Truly, this woman isn't a piece of property that her man "owns," it's a mutual agreement that she can break out of at any time. If this guy is better than her BF then she has a RIGHT to be with the better guy. If he isn't better than her BF then he won't be able to get her, no harm no foul.
Spend time with her, keep her busy, do a little kino but keep it light. Have fun but the things you do together should come nowhere close to a traditional date. Just treat her like you would any other woman (by the bible) and attraction will come. If she mentions the BF, run boyfriend destroyer on her (do a search) because it WORKS.
Don't talk about it, don't say she's hot or that you want her, or that she should dump her boyfriend. In order for this to happen she has to be able to justify it to herself. "It just happened!" if there is any premeditation on her part your whole effort will fall to pieces. In a moment of passion you kiss her, emotions explode, blah blah blah. Don't let her think. If she starts to act like you are trying to pick her up, play it cool, back off on the kino and just have fun. Fun is the key, she prolly isn't having fun with her BF anymore.
Your mouth says "friends baby! friends, friends, friends!"
your body says "Sex baby, sex baby, sex sex sex!"