if she wants to date you, she has to break up with him first-it's the right thing to do. if she wants to see you behind his back-she isn't behaving like a nice person. you don't want to date someone who isn't nice, do you? who does things behind her boyfriend's back? i think you are probably both better off without her and maybe in her next relationship she'll try honesty.......since you value your best friend and friendship lasts forever-you could also consider having someone else tell him she is doing this and you are resisting her-then he'll know you are a great friend.