What would you do for your friend?

So my friend is 19 and just has gotten his first gf. She's 15, and she's a . Im not saying this just as a general rude term, she tried to have with him while she was still dating another guy, she has tried to do numerous things with him and they haven't been dating a week. But she is depressed and chews him out all the time, has him hang around with her pot smoking friends who have gotten him to start smoking pot and he has started to drink alot. He has gotten accepted into ucla and has decided he's not going to go because he wants to stay with her in michigan. I have the power to break them up very subtly. I don't think i should though cause its not my life i've tried talking to him telling him how she's messing things up for him. So im 99 percent sure i won't break them up but what would you do if it was your friend