It's so easy hah. All you have to do is treat her better than her boyfriend does, and the SECOND he makes a mistake (even if it's a small one), exploit it as much as you can (without seeming too obvious). For example, lets say he was flirting with another girl. Talk to her about it and say something like "Guys like that piss me off, he's talking to other girls the same way he talks to you? You need a guy who treats you exclusively, not the same as all the other girls."
One argument that will almost always gain you some ground is "The sad thing is, almost all men will go to ridiculous lengths just to say he's with a girl as beautiful as yourself. Girls don't really understand how far a guy will take it. I mean, I'm a guy I know how we think, and I've BS'd girls too, but I would never do that to someone like you, only to the worthless skanky girls, who I'm not into anyway. We all make bad decisions hah. Anyway, you shouldn't take it."
^^If you can put that one together well, she just might break up with her boyfriend for no reason.