Get him a pro.

Serious. The only reason he is hitting her up is BECAUSE he knows your history with her. He prolly figures if she is willing to cheat with you then he stands a decent shot at hitting that shit as well. Get him a hooker to pop his cherry and get that shit out of his system. One that will rock his world, ya know?

SERIOUSLY though - he's thinking with his dick. It sounds like your friend isn't taking him serious if she tells you about it. Let her handle it. Remember what it was like for you at 15?