SO, there's this guy, and my friend and I are pretty sure he's an ESTP. He's close with my friend (INFJ), and he has this girlfriend (IDKW- "I don't know what" but she's the perfect definition of "evil bitch"

).
Problem is, he seems really messed up, and I'd really like some input on the matter >< of course I would relay the message to my infj friend cause she's closer to him.
He started dating this girl last year, and he reasons were: she's hot, and it's for fun. By now he's really extremely infatuated with her. However, he knows that the relationship is unhealthy, and he knows some shady things about her. He knows that she's gone really far physically with other guys, cheated on many of her previous boyfriends, and that she was going to call him out last year and dump him in front of all her friends (she's the type who dates boy after boy for a sense of security). And he acts like he doesn't care. But now, he seems to be getting almost depressed. His grades suck (he used to be obsessed with good marks), he has few friends... And the more he realizes that this is an unhealthy relationship, the more he seems to cling to it. But he's a really talented guy and I think he could really flourish if this girl wasn't dragging him down.
I hardly even know what I'm asking, but can anyone give me any insight on this? And is there any way my friend could help him realize what's happening here? He really respects her and she's one of the only people he occasionally opens up to.