My best friends crush is hitting on me, what should I do?

So right now my best friend is completely in love with this guy from our place of work, and i have encouraged her to make a move on him for ages. She is the kind of girl that falls head over heals in love for months, and then find a new one to through her love at. I actually met this dude before we started to together, and we have always had a very good connection. In class we joke around a lot, and i can tell that it bothers my best friend. Now i am not the kind of person to fall in love or anything, but from the start i have always had a kind of lust towards this guy, witch i decided to drop, when i heard my friend liked him. However now he have started to come on to me, and he is kinda teasing me for sex every time we are at a party. Until now i have turned him down, because of my friend's feelings, but i am not sure how much longer i can take this. I know that my friend really likes this guy, but i dont think she is every going to do anything about it, they never really talk at work, and she is to shy to ever approach him at parties. So the question is. Do i have a right to sleep with this guy even though me feelings are based more on lust than love? Or is her dips a dealbreaker?
My best friends crush is hitting on me, what should I do?
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