Well, depending on how much you like this girl, how much you know her, etc, you'd probably be best off resolving the situation with her and your feelings. And her boyfriend is just that, a boyfriend, they're not married.
I just know from my personal experience if you let those feelings cultivate in your mind without trying to resolve them, you'll destroy your psyche. So, you gotta decide how much you like this girl, and whether or not you can afford to ignore it or not. I'm thinking based on this post that you may like, actually have feelings for her more than "wow she's hot, she'd be so fun to screw" or something.
You pretty much gotta think for yourself on this one. How much is it all worth?
From my experience in an almost exact situation as your's, just ignoring it made things so much worse for me, I believe it may have damaged me forever. But, your experience may vary, and that's what makes it hard. Have fun.