I used to (well still can be at times but am working on it) be like this- anything that upsets me I would eat. If I was happy, I'd eat, and so on.
I finally started telling myself, am I upset or am I actually hungry?!
Okay you said your friend owes you money? Don't just ASSUME he isn't going to pay you back- how much? Have you asked him "hey okay so when were you going to pay me back? I need that money back, my kid needs.... (insert whatever here, braces, shoes, medicine). Even if she doesn't need it- just say it so he knows you are waiting for that money.
You can't control if your friend is going to pay you back, but you can control what you eat- so if you start stressing out- do something to take your mind off of it- read a book, take your daughter out for a walk, play with some toys with her, something, anything to keep your mind off of the issues.
And in the future don't let people borrow money without signing a paper that they owe you and it should indicate by WHEN they will pay you back- that way if they lag you can always take them to small claims court (just assuming you haven't had him sign a note- see if he'll sign one now).