Most Helpful Guy
Too often this happens with someone you both know. It would be easier if it was someone you didn't know. You've been betrayed by 2 people, and you don't know who is more at fault.
It can be fixable, but it depends on how likely he is to ever do this again. Only you and him can figure that out. If you leave him and find someone else, you probably have the same 50/50 chance with the next man.
I don't think your friend did you any favours. She must have given him some encouragement, and known how much this could hurt you. They usually confess only to ease their own conscience. It makes them feel better, but makes you feel worse when you find out.