You need to leave her be and move on with your life. It is true that she might like you but I reckon she doesn't dislike the other guy she is going out with either so you don't exactly have a choice here. Besides, if she is flirting with you when she has a boyfriend she is just messing with you so you should just wait and see what happens.
Do you want to be the next boyfriend? If so, get ready to deal with it when she's flirting with your friends, that is, if you still have any after you get involved with a friends girl.Trust me, you don't want to be that guy.