If she has someone you shouldn't; that's just not a cool thing to do. The side of the coin that people don't want to think of is: if she goes with you now, she's cheating. Do you honestly want someone you know will cheat?
Talk to her a lot and be around her alot. And then her bf will get jealous and they will fight and break up and she'll come to you for comfort. Then tell her how u feel. Just dont tell her that u broke them up