How to win a girl over her current boyfriend?

This girl is in a dysfunctional relationship with her boyfriend (they've been going out for seven months), and I want her to be with me. They get into arguments often about who they're hanging with, flirting, etc. but they end up reconciling and trying to work things out. She says that he wins her heart back with the things he says. It's been a roller coaster for them. I want to convince her that what she is doing is completely irrational and that he could be taking advantage of her. And also..I'd like to have her, though I find that wrong as well. It's a war between my mind and my heart...Help!