First time DM, and couldn't find the answer. Here's the scenario that happened:

Evil wizard is fleeing from a PC while the rest of the party is occupied. Casts suggestion on a Friendly NPC: "You're friend is going to get hurt if she follows me. Please hold her off". NPC grapples PC. PC spends a few turns trying to escape and talk him out of the suggestion. Eventually gets out, and starts attacking the NPC.

Does the suggestion now become an "Obviously harmful act" and not work, or because it was not harmful initially it still carries? 

The spell does not say that the spell breaks if the task become harmful, so it doesn't. It only checks that clause when you cast the spell.

You could argue that after a turn or 2 of "holding" the suggested task has been completed.

Names are handy to reduce confusion. Let's give them some.

Bob the Evil Wizard is fleeing from Sara while the rest of the party is occupied. Bob casts Suggestion on Joe: "Sara is going to get hurt if she follows me. Please hold her off."  Joe grapples Sara, and Sara spends a few turns trying to escape and talk Joe out of the suggestion, eventually gets out, and starts attacking Joe.

First up, Joe gets a save. Even if he fails it, he still knows that something was done to him, he's going to be suspicious of Bob later.
Next, Is Sara one of Joe's friends? If she isn't, Joe is just going to stand there, waiting for some clarity. or maybe chase after Bob to get it.
Next, If Sara doesn't happen to be one of Joe's friends, who does he grab? Unless there is no other female around, and if Sara is wearing armor it might be hard to tell if she is or not, what will Joe do?  He's already suspicious of Bob, he'll probably go after him.
Next, let's go ahead and say that Sara was Joe's friend. He grabs her and he holds her until she starts to struggle. How long do you hold a struggling friend? I think Joe would let her go immediately. Sara is a friend! If she's struggling she must not care if she gets hurt, and she's a friend. Joe has his doubts about Bob, the most likely thing for him to do is help her chase Bob.

Your version said that once Sara got free from Joe, she attacked Joe. Why would she do that? The one she is after isn't Joe, she's after Bob! Joe didn't hurt her, he just tried to slow her down.

So lets go way back to the beginning. Sara is chasing Bob. What does Bob do? He wants to get away, so what's the best way to do that. Stop and cast Suggestion, or cast Expeditious Retreat while still running as a Bonus action? He can't use both, they each require Concentration. I know what I would do if I wanted to get away.  If I'm an Evil Wizard, If I'm going to stop to do something, I'll cast Fireball at Sara.  I don't care if anyone else gets cooked, because I'm Evil after all.

Quote from DxJxC >>

You could argue that after a turn or 2 of "holding" the suggested task has been completed.

 I was fully prepared for her to say "You have held me off. Mission accomplished" and end the spell that way, but I got worried when she started attacking. Suggesting someone do your laundry is reasonable. But does it still hold if a dragon attacks the laundromat?  RAW it seems like the answer is yes. 
 

If I'm an Evil Wizard, If I'm going to stop to do something, I'll cast Fireball at Sara.  I don't care if anyone else gets cooked, because I'm Evil after all.

Saying "The wizard should've cast Fireball and Expeditious Retreat" Is not especially helpful advice. 

Quote from Unagi776 >>

First time DM, and couldn't find the answer. Here's the scenario that happened:

Evil wizard is fleeing from a PC while the rest of the party is occupied. Casts suggestion on a Friendly NPC: "You're friend is going to get hurt if she follows me. Please hold her off". NPC grapples PC. PC spends a few turns trying to escape and talk him out of the suggestion. Eventually gets out, and starts attacking the NPC.

Does the suggestion now become an "Obviously harmful act" and not work, or because it was not harmful initially it still carries? 

No.  Suggestion ends if you ask the target to commit an obviously harmful act - if you ask the target to perform an act which is not obviously harmful and then leave, and then circumstances change so the act becomes obviously harmful, the fact remains that the target was not asked to commit an obviously harmful act.  Obvious harm, whatever that means, only matters at ask time, not at perform time.  Similarly, supposing it's not obviously harmful to suggest someone to shout "Kurtulmak is pregnant with Garl Glittergold's babies!" into a kobold bar down on the docks because they have no idea who those two entities are, it won't break the spell if the target later learns exactly what it is they've been suggested to do.

If you didn't find my suggestion helpful, how about this?  The spell fails if the suggestion does not sound reasonable.  Is it reasonable to ask someone to grab someone who is armed and armored and running along chasing someone?  No.  Spell fails.  Let's suppose that Joe has less than average mental stats and the spell takes effect.  Joe grabs Sara.  The instant her grabs her if she starts to struggle, he gets a save because no matter how low his mental stats are, you don't hold on to a struggling friend who is armed and armored, someone is going to get hurt.  He will keep on making saves every turn until he makes one.

Quote from Geann >>

If you didn't find my suggestion helpful, how about this?  The spell fails if the suggestion does not sound reasonable.  Is it reasonable to ask someone to grab someone who is armed and armored and running along chasing someone?  No.  Spell fails.  Let's suppose that Joe has less than average mental stats and the spell takes effect.  Joe grabs Sara.  The instant her grabs her if she starts to struggle, he gets a save because no matter how low his mental stats are, you don't hold on to a struggling friend who is armed and armored, someone is going to get hurt.  He will keep on making saves every turn until he makes one.

 Suggestion does not actually have to be a reasonable request. It only has to sound like one, and the specific example used, (Give your warhorse to the first beggar you see) is an extremely unreasonable thing to ask for most people. In this case "Joe", who is an armed and armored Veteran and failed his wisdom save grabbed "Sara", the non-armored Wizard. 

Now if you're suggesting Joe should get a wisdom saving throw each time he encounters resistance, I'd say that's a pretty reasonable houserule that I might adopt in the future, but the spell doesn't describe that the way Hideous Laughter or Hold Person do. 

Details do indeed come in handy.  I probably wouldn't have argued much if I hand known some things.  Why was Bob running away?  What was Joe?  An armed and armored veteran wouldn't have any problem holding a Sara for as long as he could if he saw a friend with no armor running around after someone.  It would also be handy to know why Sara (apparently alone) was chasing Bob in the first place.  It's kind of dangerous to chase a Wizard, and being one herself, Sara would know to bring her friends along.

As for the thing about a Paladin's horse?  On that we agree completely.  That's totally unreasonable.

I would say that in this scenario, the NPC is clearly befuddled by the spell into doing as the evil wizard asks. If the PC said anything like "let go of me!" then the NPC would just say "oh! oh, ok!" and let go of them.

As for an unharmful act which becomes harmful, that's fair game I think. If you use suggestion to say "can you get the milk from the fridge", and the NPC doesn't know the fridge is full of rabid bees, then they will walk over and open the fridge, and be stung a lot, possibly getting rabies. The action they requested wasn't harmful.

I think of it as if you said "jump off the cliff", they would just say "um, no, I'm not doing that!". If the target knows it will be harmful, they won't do it. If they don't know (EG they roll low to find traps and you ask them to walk down the corridor where you know there are traps) then they will do the act they see as harmless, and then get hurt because of it.

This is why I rarely bother with suggestion as a DM or a player....the "reasonable" thing is just bad design/language.  Who decides what is reasonable? Does the target? Does the caster?

A paladin may have an oath to never give up his blade so if the suggestion is "That sword is heavy you should set it down for a bit" they would see that as completely unreasonable as they are in combat and they would never give up their weapon.  

But you ask a militia man who is old and tired....then yeah seems reasonable to them.  You have to interpret it for each creature which makes it kind of a headache. 

There is no metaphysical quality to a course of action that makes it harmful or unharmful…only context. “Steal that man’s hat” is not harmful usually… unless that man is the king surrounded by some scary looking guards, in which case it’s suicidal. I see no problem with letting a suggestion end when evolving context causes the formerly unharmful course of action to become quite dangerous. “Pick up what is inside that box” could get an NPC to spend a round opening a box, but probably not a second round picking up the venomous  snake inside.

Quote from Geann >>

Names are handy to reduce confusion. Let's give them some.

Bob the Evil Wizard is fleeing from Sara while the rest of the party is occupied. Bob casts Suggestion on Joe: "Sara is going to get hurt if she follows me. Please hold her off."  Joe grapples Sara, and Sara spends a few turns trying to escape and talk Joe out of the suggestion, eventually gets out, and starts attacking Joe.

First up, Joe gets a save. Even if he fails it, he still knows that something was done to him, he's going to be suspicious of Bob later.
Next, Is Sara one of Joe's friends? If she isn't, Joe is just going to stand there, waiting for some clarity. or maybe chase after Bob to get it.
Next, If Sara doesn't happen to be one of Joe's friends, who does he grab? Unless there is no other female around, and if Sara is wearing armor it might be hard to tell if she is or not, what will Joe do?  He's already suspicious of Bob, he'll probably go after him.
Next, let's go ahead and say that Sara was Joe's friend. He grabs her and he holds her until she starts to struggle. How long do you hold a struggling friend? I think Joe would let her go immediately. Sara is a friend! If she's struggling she must not care if she gets hurt, and she's a friend. Joe has his doubts about Bob, the most likely thing for him to do is help her chase Bob.

Your version said that once Sara got free from Joe, she attacked Joe. Why would she do that? The one she is after isn't Joe, she's after Bob! Joe didn't hurt her, he just tried to slow her down.

So lets go way back to the beginning. Sara is chasing Bob. What does Bob do? He wants to get away, so what's the best way to do that. Stop and cast Suggestion, or cast Expeditious Retreat while still running as a Bonus action? He can't use both, they each require Concentration. I know what I would do if I wanted to get away.  If I'm an Evil Wizard, If I'm going to stop to do something, I'll cast Fireball at Sara.  I don't care if anyone else gets cooked, because I'm Evil after all.

That may be how you run Suggestion but that is not what is written in the book so expect other DMs to run it differently. .

SUGGESTION: Duration: Concentration, up to 8 hours

"You suggest a course of activity (limited to a sentence or two) and magically influence a creature you can see within range that can hear and understand you. Creatures that can't be charmed are immune to this effect. The suggestion must be worded in such a manner as to make the course of action sound reasonable. Asking the creature to stab itself, throw itself onto a spear, immolate itself, or do some other obviously harmful act ends the spell.

The target must make a Wisdom saving throw. On a failed save, it pursues the course of action you described to the best of its ability. The suggested course of action can continue for the entire duration. If the suggested activity can be completed in a shorter time, the spell ends when the subject finishes what it was asked to do. You can also specify conditions that will trigger a special activity during the duration. For example, you might suggest that a knight give her warhorse to the first beggar she meets. If the condition isn't met before the spell expires, the activity isn't performed.

If you or any of your companions damage the target, the spell ends."

In this case, the wizard suggested to an NPC (presumably NOT their ally since then a suggestion would not be needed - likely an ally of the PC).

""You're friend is going to get hurt if she follows me. Please hold her off."

First question, is this a reasonable suggestion? As DM, I'd say yes - it makes sense. The target of the spell gets a wisdom saving throw. If they fail then the spell takes effect.

The NPC acts to prevent the PC from chasing the wizard by grappling them and likely telling them it is too dangerous, "if you follow the wizard you will get hurt". The NPC continues to try to grapple or hold off the PC as long as the PC is trying to follow the wizard.

There are NO extra saving throws. The character affected by the suggestion is affected until concentration ends. They will try to fulfill the suggestion until it is completed. In this case, as long as the PC tries to follow the mage the NPC is going to try to stop them.

The PC escapes the grapple and turns on the NPC friend/ally and begins attacking them. At this point the NPC is unlikely to attack their friend, they are going to yell at the PC to stop attacking, they will ask "why are you attacking?", the NPC will say that "they are only trying to help and that it is too dangerous for the PC to follow the mage".

If the PC continues to attack the NPC, the NPC might defend themselves or they might dodge. However, they will likely stay close enough to the PC to prevent them from following the wizard if they change their mind. The NPC only has to act when the PC follows the mage and even then they only have to "hold" them.

Taking damage in this case does not break suggestion since it is not caused by an ally of the wizard but by an ally of the NPC.

Does the suggestion end when the PC breaks the grapple and is no longer held? Or does it end if the PC pretends to stop following the wizard? As a DM, I would say no (others may have different opinions) - as long as concentration is being maintained and it is still possible for the PC to follow the wizard then the NPC will try to hold the PC if they try to follow the wizard. I think it is a good use of the suggestion spell by the wizard :)

Either way there are no extra saves, there is no confusion about who to stop (assuming it was clear from context WHO the wizard wanted stopped when the spell was cast), and in my opinion the spell will continue as long as the wizard concentrates and it is still possible for the PC to follow them. However, the NPC won't try to hold the PC unless it is clear that the PC is trying to follow the wizard.

P.S.

1) Fireball is only a useful escape spell when the target is guaranteed to die on a successful save. If they aren't going down there are better options. Assuming that the wizard is 5th level and has spell slots left.

2) In this case, the wizard had two close pursuers - the PC and the NPC - Suggestion managed to delay both and only required one saving throw. It is also available as a 2nd level spell minimizing resources.

David42, you obviously didn't actually read anything past the post you just quoted, and that was post #4 in this thread.  Go back read past the fourth post, there aren't very many of them.

Quote from Geann >>

David42, you obviously didn't actually read anything past the post you just quoted, and that was post #4 in this thread.  Go back read past the fourth post, there aren't very many of them.

 I read the rest before I posted :). I didn't find any of your other posts correcting the "errors" in your first post nor did they really add much.

Quote from Geann >>

If you didn't find my suggestion helpful, how about this?  The spell fails if the suggestion does not sound reasonable.  Is it reasonable to ask someone to grab someone who is armed and armored and running along chasing someone?  No.  Spell fails.  Let's suppose that Joe has less than average mental stats and the spell takes effect.  Joe grabs Sara.  The instant her grabs her if she starts to struggle, he gets a save because no matter how low his mental stats are, you don't hold on to a struggling friend who is armed and armored, someone is going to get hurt.  He will keep on making saves every turn until he makes one.

 "Is it reasonable to ask someone to grab someone who is armed and armored and running along chasing someone?  No."

Clearly we disagree. The OP mentioned that the PC and NPC were friendly (I assumed the NPC was friendly to the PC rather than the wizard which seemed reasonable.

Asking a character to stop their friend from hurting themselves by chasing after a nasty wizard by holding on to them actually sounds completely reasonable in my opinion as a DM. They may have armor and weapons but the NPC doesn't expect their friend to use them and (due to the suggestion) the wizard is perceived as dangerous to the PC whether they are a heavily armored paladin, a monk, a sorcerer or whatever.

You also clearly did NOT read my post or the text of the Suggestion spell posted. The creature affected does NOT get another save (RAW). They are affected by the suggestion, it doesn't matter if they struggle with an armed character or an unarmed one. The NPC in this case thinks that they are acting in their friend's best interest preventing them from getting hurt chasing the evil wizard. You can run your game however you like but RAW there are NO additional saves against the effect of the suggestion.

"He will keep on making saves every turn until he makes one."

This is a house rule. As I said, in your game you can run it however you like but most folks asking questions here want answers based on rules as written.

Quote from Geann >>

Details do indeed come in handy.  I probably wouldn't have argued much if I hand known some things.  Why was Bob running away?  What was Joe?  An armed and armored veteran wouldn't have any problem holding a Sara for as long as he could if he saw a friend with no armor running around after someone.  It would also be handy to know why Sara (apparently alone) was chasing Bob in the first place.  It's kind of dangerous to chase a Wizard, and being one herself, Sara would know to bring her friends along.

As for the thing about a Paladin's horse?  On that we agree completely.  That's totally unreasonable.

Honestly, all of these extra details are completely irrelevant. They do not affect the casting of the suggestion, the saving throw or the effect on the NPC and PC. There is a lot of DM discretion in determining when a suggestion is reasonable - but none of these details matter in this case. Whether Bob is running away because he just killed someone, is afraid, or just happens to want to get away from the party doesn't affect the spell or the save in any way. Similarly, the classes of Joe and Sara, how they are dressed, what their motivations are also don't matter. 

Is it reasonable for one friend (the NPC) to look after another friend (the PC) by preventing them from doing something dangerous by holding on to them? I'd rule that reasonable under almost any circumstance - what else are friends for but to look out for each other?

After deciding the suggestion is reasonable, the rest plays out without any further saving throws (the suggestion spell only lists TWO end conditions - concentration and damage being done by the allies of the caster).

You base your assumption on something not stated and in the face of it, unlikely.  That Joe and Sara are actually friends.  What if they are not?  The spell tells Joe that she is, so right then and there, that's not reasonable.  The DM gets to decide how much so, but it's something that has to be known or questions cannot be properly answered.  The issue of whether or not Joe or Sara was the one armed and armored makes a big difference.  I figured that Joe was not, and Sara was.  Turned out to be the other way around.  Once I knew that, I agreed that Joe would grab hold of Sara and try to slow her down.  How long is now up to Joe.  He's holding someone he believes to be a friend.  What if she struggles?  

Myself, if I was Joe, I'd let go the second Sara shouted.  I apologize to my friends if I do something to them that makes them shout.  You may be different.

Quote from Geann >>

You base your assumption on something not stated and in the face of it, unlikely.  That Joe and Sara are actually friends.  What if they are not?  The spell tells Joe that she is, so right then and there, that's not reasonable.  The DM gets to decide how much so, but it's something that has to be known or questions cannot be properly answered.  The issue of whether or not Joe or Sara was the one armed and armored makes a big difference.  I figured that Joe was not, and Sara was.  Turned out to be the other way around.  Once I knew that, I agreed that Joe would grab hold of Sara and try to slow her down.  How long is now up to Joe.  He's holding someone he believes to be a friend.  What if she struggles?  

Myself, if I was Joe, I'd let go the second Sara shouted.  I apologize to my friends if I do something to them that makes them shout.  You may be different.

 

 As I've said several times, in your game, you can run it however you wish.

In the OPs posts, Joe and Sara appear to be allies or at least on the same side if not actual friends (since the OP states that the veteran is not on the same side as the wizard). In my opinion that is sufficient to get them to act to prevent harm from coming to their allies. In addition, the suggestion only asked them to hold their ally. It didn't ask them to harm them or do anything extraordinary. This also seems quite reasonable in context. Finally, whether the wizard grabs the fighter or vice versa, it is not unreasonable to grab hold of your ally/friend to prevent them from going into danger - the only difference is that the wizard is far less likely to be successful at holding the other.

In addition, you really don't seem to understand the Suggestion spell as written. "Myself, if I was Joe, I'd let go the second Sara shouted" ... the suggestion was to hold the ally as long as they were following the wizard to save them from the danger. The person shouting doesn't, in any way, change the suggested course of action. If the PC attacks the NPC then, RAW, it still doesn't change anything this since the action has to be obviously dangerous when the spell is cast. However, I myself might house rule either the spell ending or another save if a situation became obviously dangerous due specifically to the suggestion (Like Champs example of opening a box with an obviously poisonous snake with the suggestion to pick up whatever was inside the box) but I realize that is completely a house rule. Normally, Joe would try to "hold" Sara back. However, if Sara is trying to fight Joe then they aren't chasing the wizard and the suggestions is working since she is effectively being "held" from chasing the wizard - so maybe Joe would dodge and try to talk Sara out of attacking them as long as they didn't chase the wizard :)

If your friend tried to run into a burning building trying to save something from the Fire and you were holding them back from pretty good odds they get hurt or die, would you let go at the first signs of struggle or shouting?  I guess it might depend on what they are trying to save. But knowing something harmful is going to happen I would think the NPC would keep holding until the spell ends. 

Ok, so here's where things get really confusing...

At the point where the PC turns on the NPC and begins attacking them, they are no longer the friend of the NPC. The NPC will no longer feel the need to protect the PC by holding them back, because if that PC is trying to stab them with a dagger, what do I care if they get hurt? Just how much love does the NPC have for the PC that they will keep grappling them?

The wording of the original suggestion relies on the NPC believing that they need to do it to protect their friend. So if the PC simply says "I licked all your dinner before I gave it to you! Aha!" then the request is no longer a reasonable one.

Fun times.

Ultimately, this is why the rules aren't really all that important and should always be taken as general suggested guidelines, never as set in stone.

Quote from Sanvael >>

Ok, so here's where things get really confusing...

At the point where the PC turns on the NPC and begins attacking them, they are no longer the friend of the NPC. The NPC will no longer feel the need to protect the PC by holding them back, because if that PC is trying to stab them with a dagger, what do I care if they get hurt? Just how much love does the NPC have for the PC that they will keep grappling them?

The wording of the original suggestion relies on the NPC believing that they need to do it to protect their friend. So if the PC simply says "I licked all your dinner before I gave it to you! Aha!" then the request is no longer a reasonable one.

Fun times.

Ultimately, this is why the rules aren't really all that important and should always be taken as general suggested guidelines, never as set in stone.

The compulsion of the magic doesn't care if your opinions changed after the initial casting. It simply compels you to follow the order that was given at the moment of casting. The spell description includes the triggers that will break the spell and no longer knowing or believing the original suggestion is not one of those.