Just talk to your best friend. If your best friend was truly your friend, she would understand and decide to back off a little with the flirting. If she happens to also have a crush on him, then the two of you should decide on an agreement about how you should settle this. This agreement should be something simple and not competitive since competition can ruin friendships. It can be something as simple as you both agree to stop liking this guy and try flirting with other guys. If she decides to flat out refuse to do anything about this, whether it is backing off or coming up with a compromise, then you know that she's inconsiderate to your feelings and therefore not a good friend to you. From then you can just break things off with her and find better friends.