Personally i'd just keep doing what i was doing, if it's getting you results. Resist yeah and don't fully do anything with her, but definately keep doing what is getting her interest. Then over time she's bound to follow how she feels, something will break what is holding her back and she'll finish with him and get with you... it's what i've done in the past, and it is the natural way. You didn't steal her, you just were the better guy.
As for all this BS about plenty of single women out there... find one, and i bet you'll find she has feelings for a guy who won't go out with her. Yeah she's single, but it's the same deal with having to overcome competition. And even if there is a guy, so what? Boyfriend, it's just a word, it means nothing. All that matters are the feelings, how she feels for him. Just as there are plenty of single women, you'll find plenty of women who are with a guy just because they don't want to be alone. Plus half the boyfriends out there are complete idiots, they don't deserve the women they are with.