You can let a girl know you like them without making a move or implying that they should ditch their current boy friend for you. I've been there, and on a slight twist, I've had a girl do that to me while I was seeing someone else. If I was single at the time I'd have taken her as well. If we we're both single now I would, but I wouldn't chose her over who I'm currently with. What she said didn't make me think she was asking me to chose her over the girl I was already seeing, she was just letting me know she liked me. It can be nice to know someone likes you, even if there is no chance of something happening between you both.
I would agree with your last comment though, it's not healthy to become infatuated with someone you can't or rather, shouldn't have. Imo I'd let her know, but I wouldn't get infatuated with her, I'd keep her at a freidnly distance and if she became available before you meet someone else then you can take things further but don't count on that if too much. Also following on from TM's comment again, I most certainly would not get involved in what's going on with her and her b/f.
At the end of the day 32' Lover, you know her better than any of us, so while we're all posting what we say we'd do your the one who knows her best. Read the different pieces of advise, use your judgment to what you feel is right on both your part and more importantly, hers.