My friend is trying to date a 12 year old, PLEASE HELP!!!?

My Buddy is trying to get with a 12 year old! hes 18 and I'm 18, were both dudes, and hes been confiding feelings for her to me for a while now and now hes making moves on her, today he KISSED HER! Please somebody help, I'm trying to get him to stop without calling the police but I'm not sure what to do to get him to back off/find a girlfriend his age! Also would I get into trouble for being an accomplice if I didn't inform the authorities? I'm pretty sure an 18 year old making out with a 12 year old is EXTREMELY illegal!