Just remind him that it's statutory rape - & he'll be a sex offender for life, on the record. It will ruin his life. Possibly hers, too. And since he knows what he's doing, he can't claim she lied to him about her age. He needs to be with women his age. 15 is borderline pedo territory for an adult.
Theres really nothing you can do, other than try to advise him and talk some sense into him. At the end of the day they both are going to do what they want, If he gets caught then he is going to wish he listened to you..God bless :)
Hmmm.... I'm going to have to say to tell her dad or his parents. And they might wind up having sex, and that's going to be your friends fault if she gets pregnant. I don't care if they use condoms, it's a sin! You should threaten to say you're not going to be his friend anymore, explain to him WHY it's wrong, tell both of their parents, tell your parents, and that's all I got. I honestly think all that stuff is really gross!!! :-( Hope it helped!!! :-)