The first and the fairest thing you should do is split with your current boyfriend. Its cruel and unfair to keep stringing him along if you no longer have feelings for him.
I think that you should then have some time on your own, to let things settle and to see if the feelings you have for your b/f's best friend are still there (on both sides)..
If you do then decide to get together with him, have you thought about the possible consequences? Losing your best mate would be horrible, so you would need to be sure she was ok with the situation. Could you talk to her about it?
Its a difficult situation, you cant help who you fall for and if handled sensitively enough, it will work out in the end.