Before you take any decision, be sure that your friend is actually flirting with your boyfriend. Take out time to examine the whole situation. You do not want to take any decision that you will regret in the future.
Talk to your boyfriend about it, stick to the facts you have gathered and not go on a tangent. Give him time to respond as well. You can also talk to your best friend, let her know that you might not know what she means by her actions but if she is trying to hit on your boyfriend, you think it’s not good. Let them know you are uncomfortable with what’s going on between them.
Finally, do not try to flirt with someone else in the presence of your boyfriend, as a means to get back at your boyfriend; this will only make you look cheap and desperate.