Stay friends with her. The whole "once you're friends you'll never be anything more" is a load of crap. I would never go for any guy whom I do not know. One of my housemates and her fiance were almost best friends at uni for about 3 months before they got together. Besides, why should you lose a perfectly good friendship just cus you can't date her?
Secondly, no, don't ruin their relationship. You'll get yourself into too much. There will come a point where either you're embarrassed cus she rejects you, or she starts getting confused and end up hurting you and her bf since she's undecided as to who to go for as she fancied both. And trust me, when someone you like/love gets confused about the relationship, it's hell. (been there). One of my friends flirted with this girl who has a bf and she even said she'd break up with her bf after exams, and she never did and never even said anything or explained to my friend. Listen, if she's so interested in you, she'll lose interest in her bf, and she would break up with him even without you flirting with her. Then when she's single, make whatever moves you want on her.