In a recent session a player used the friends cantrip to prevent an enemy before it became hostile. Before the one minute session ran out he cast it again. And again. And again. I didn't question it at the time because it was benefiting me and the DM's reaction was hilarious, but now I have questions.

Does Friends really not have a saving throw at all?

Can it be cast over and over without restrictions like that?

RAW It only works one time. When the spell ends the creature becomes hostile towards you and it only works on targets that are not hostile towards you. 

It’s a great spell for a trickster of some sort. Using it when you can change your appearance or to trick someone into letting their guard down so that you have advantage on your first attack are two great uses for it. 

Friends does not have a saving throw, correct.

But note that RAW, all it does is give you advantage on charisma checks. Any other effects are entirely up to the DM. It does not provide a way of "preventing an enemy before it hostile" unless the DM wants it to.

Also, it's important to remember that (barring certain sorcerer abilities) spell casting is OBVIOUS.

In your example above, the person the character is interacting with would be, like "Hey, we're having such a nice conversation. I really like you. Why are you daubing makeup on your face and saying those funny words?" (rolls Insight check)

Friends doesn't have a Verbal component, only Somatic and Material.  

Quote from Tim >>

Friends doesn't have a Verbal component, only Somatic and Material.  

 Yes, correct. But you get the point.

If you have an arcane focus you can ignore the Material component. Only the Somatic component is supplied which is not defined so you can rule the Somatic component is just tracing a rune in the air, which can be done with your arm down by your pocket out of sight, or under a table or something. 

The spell can be used over and over again. But it is concentration so the instant you try to cast it again, the previous casting instantly ends and this means the creature you had targeted for the effects of the spell will become immediately hostile to you. The second casting still technically applies - the target is "self" but you no longer get advantage on Charisma checks against that creature. 

 So short answer: No, you can't keep charming the same person like that.

Quote from Cyb3rM1nd >>

If you have an arcane focus you can ignore the Material component. Only the Somatic component is supplied which is not defined so you can rule the Somatic component is just tracing a rune in the air, which can be done with your arm down by your pocket out of sight, or under a table or something. 

The spell can be used over and over again. But it is concentration so the instant you try to cast it again, the previous casting instantly ends and this means the creature you had targeted for the effects of the spell will become immediately hostile to you. The second casting still technically applies - the target is "self" but you no longer get advantage on Charisma checks against that creature. 

That's what I thought, but I had to check.

Would you say that you need line of sight on the chosen creature at the time of the casting? The range is self, so would I be able to hide around a corner and cast the cantrip. Then once head into the room, choose the creature that I want to have advantage on?

You answered yourself. Range is self, and there's no other requirements about the other creature presented in the spell other than that you choose it at the time of casting, so you don't need line of site or line of effect to anyone other than yourself. It might be a little ambiguous whether you can cast Friends to give yourself advantage on checks against "the next creature that walks through that door" or "the next guard I see" or something like that, rather than knowing who that creature is.... but there certainly isn't a line of sight requirement. 

But also, Friends is just a poorly edited spell. Despite targeting only yourself, in theory if you cast Friends but then never so much as speak to the other creature (indeed, even if you don't meet the other creature), when the spell ends that creature becomes hostile to you anyway and knows you tried to influence it. Blehh, spells do what they say they do, but what Friends does makes no sense.