ITB???
Seriously though, from the sounds of it, you’ve taken the best course of action: Get the kid that did it to man up.
I can’t really say what I would do in this situation becuase I’ve run into this a few times and leaned both ways depending upon the circumstances. He’s a friend and you should tell him, but then again, it might be better to let him learn for himself (its usually a better lesson that way). Not easy, which is why I usually try to stay away from drama like this.