It sucks but she likes you so much because your friends with this guy. She gets to talk about him all she wants and you sit there and listen. You probably even give her advice and your opinion on the whole matter.
First you need to reduce the amount of time you spend with her AND when you are around her, try to stop her from talking about him. Bluntly change the subject. If a girl starts talking about some guy to me I usually just blankly stare at her until she pauses and then I change the subject to something lame to show her how much I didnt care about what she said. "The weather looks nice outside, dont you think?"
Secondly you need to know that this girl does not like you. She likes your friend. She calls you her best friend because thats what you are, a friend.
The only way I see her ever liking you is if you stop focusing on her and get other girls. Its not stressed enough that girls want what other girls want. If she hears that ectomorph is talking to mary jane then suddenly she may find you attractive.
This girl goes on the back burner for now. She is what you should call long term game. You must be patient and never let her know you like her. When you make a girl long term game you need to wait until she comes to you. She needs to give you the green light, pretty big IOIs. Misjudging her and prematurly making you move will ruin your chances with her.
And long term game will take a long time(duh), she will need to see changes in your, small and slight but attractive changes. How you talk, how you hold yourself, your social life, how well you communicate, how mature you are, etc. No less than 2 months you can make this happen. It can take as long as a couple years.
Btw read up on body language and alpha male attitude.