In a magic shop (of sorts), there is a robe that looks cool. I really just put it in there to fill out some of the space, but, stupidly, I didn't expect my wizard PC to inquire about it. The shopkeeper told her that it is a "robe that mends mortal wounds". I kept it vague to allow for some maneuverability. I gave her a price, and she is rapidly getting the money to buy it (she is not great at staying alive, so this appealed to her). I need assistance in creating a new item that isn't too powerful, and won't be disappointing.

Here's the facts of it:
Its a robe that is iridescent blue/black with gold inlays. It mends wounds. It costs 850gp. 

Any ideas on what I could make it? I currently have a robe that reduces KO hits by 1d4 per level, once per long rest. That seems like it will get OP when we get to higher levels.

Thanks y'all! Here's to my first DM campaign going well!

Why not make it a bit of a cursed boon?

Robe of Life Stealing: whenever the player activates this robe, by speaking a command word, they are healed for no more than of half their maximum HP total. In a 5ft area around the user, starting with the creature that has the most current hit points takes 1 damage. The damage will continue until the total HP pool has been refilled. Each 5ft square of live vegetation around the user is also destroyed counting as 1 hp.

"Mortal Wounds" is going to be your difficult area.  If I heard that, I would be thinking it would at least cast Heal (6th level spell).  But then you say it is only going to be 850 gp.  At best that is a low tier Rare (blue) item.  That precludes any spell like Healing.  Though, you could possibly reskin a Periapt of Wound Closure to do a similar thing.  Though as that is a green item, it would be a bit expensive as that is typically a 500 gp max item.  You could make it so the wearer could change certain aspects of the robe (color, etc. though you seem to have that locked in) or make it so that the robe magically mends itself any time it takes damage (1 hr / 10 points damage you received or something of the sort)... this might be a fair compensation for the additional cost.

Item prices are more a guideline then they are a rule.  I think based on the wording, a +1 robe with an enchantment identical to the Periapt of Wound Closure would fit your bill.  It handles mortal wounds, and would increase her AC making her slightly less likely to take the hits that are killing her to begin with.

I feel like in situations like these, my temptation is to get all DM-y and start crunching numbers on price, rarity, etc. But really, it's less about the hard coin count the player is going to have to cough up to buy the thing and more about what they're conceptually spending. In other words, it's less about the actual price tag and more about the relative cost to the player. 850 gp is nothing to a player that has amassed some wealth (regardless of what level they've obtained). I see something I like and boom, I slap some coin on the counter and leave with it. In situations like that, the stakes aren't quite as high with regard to the item's features. If I don't love the thing, it'll sit in my pack for the rest of my campaign and I might forget I even have it until I need a bit of cloth to MacGyver my way through some weird trap situation or whatever. I sure as hell won't mind having laid down coin for it...I knew what it cost and I bought it.

My point is that if your player is skrimping and saving for that super-awesome thing they saw in some shop, then in the interest of maximizing the PC fun factor, that item should be pretty damn sweet when they finally pick it up. I'd err on the side of OP, given the sacrifices the player is making to obtain the thing (again, less about the actual cost and more about the relative cost).

To throw a concrete idea at you: this could be a "drop to 0" effect, automagically triggering a standard-issue Cure Wounds on the wearer, once per X number of days, and a fixed bonus to death saves. Then you could add further flavor with one or more reskinned effects pulled from XGtE's Common Magic Items, especially if those added common magic item features specifically suit the playstyle of your wizard. I am in love with the Cloak of Billowing and I couldn't care less that it provides no mechanical benefit whatsoever. Strike a straight-up Captain Morgan pose and set that thing to billowing dramatically and tell me you don't instantly feel better about your life. That section of Xanathar rocks so hard. When I first read it, it seemed like a big "who cares?" But the reality is that those common magical features give you a mechanically safe way to throw any number of magic items and/or boons at your players without altering gameplay in any major way. The net result is a way to make an item feel way cooler, more meaningful, and more memorable without having to sweat the impact to your game too much.

A lot of mentions of Periapt of Wound Closure in this thread.

Personally, my first thought would be something that during the players bonus action, it heals them a small amount (or they can always take an instant or bonus action to heal with the item). This could also stabilize on their turn, again similar to Periapt of Wound Closure as has been mentioned above. 

By the wording:

"robe that mends mortal wounds"

Nothing else, nothing more.

Any hit that brings the wearer to 0 Hit Points is instantly healed the same number of hit points as the injury (literally mending that specific mortal wound). While this keeps the wizard in the fight, it may also provide something that might just justify the price tag. The item was created to keep a torture subject alive regardless of what he is put through. "Tell us where the gold is." Stab in the neck. Healed. "Tell us where the gold is." Stab in the neck..... You get the idea. From the description it could be drow craftsmanship too. That would leave you an out as to why it does not give any save or AC bonuses etc.

How about my homebrew.

"Tharngard captains sword"".

It's a sword that can cast magic. :)

Look at my homebrew for more info.

Feel free to adjust to your needs. Smiley Face.