What did she mean by that?

This girl likes the same guy as me (my best friend) but she keeps denying it (because she wants to be a good friend and she knows I like him) but I kinda feel bad for her because she's liked him since before I knew him and never got the chance to go out with him and whenever I bring her up in the conversation with him he always changes the subject and the thought of going out with her never gets into his mind. I was talking to her on msn and she said that when I move (I'm moving during Christmas break) she wont be friends with him. She said she's only friends with him because I'm friends with him. I don't get it. Why wont he take her into consideration? Also his friend told me that he likes me but he's too shy to ask me out. If he does get the courage to ask me out should I go out with him? I really like him and everything but I know what it's like to be that girl who sits on the sideline and watches your crush go out with another girl. I really don't want her to feel that way. How can I get him to see her as a possibility of being more than a friend? I kinda feel guilty because I don't even try to flirt with him like she does and he likes me and she's been trying so hard to get him. What do I do? Do I go out with him or do I get him to go out with her?