Most Helpful Guy
You can try and talk it though with your best friend! I'm in a similar situation and me and my best homie talk about anything and everything! I can't guarantee that by talking it out things will work out for you, but being honest with your friend can't hurt you. Just tell her how you feel about the whole situation and see what she has to say!
Most Helpful Girl
It really depends on who you value more: This guy, or your friend.
You could be honest with your friend and see what she does, or you can make a move on the guy first but potentially ruin your friendship. Unfortunately, since he isn't yours and you haven't a made a move, it's not your place to be upset with her, especially if she doesn't know.