Well you really shouldn't! You should be happy that someone likes you. But if you feel like they are to close to you but you like someone else just say that you like someone else and tell them how you feel, and that you just wanna b friends.
I think that you shoulld talk to your friend and if she really likes this guy then let her have him, there is more fish in the sea. there is NO point in fighting with your friend or both of you should find someone else