If your best friend is flirting with your boyfriend, then personally pull her aside from all of others and talk to her. If they don't listen to you, say you know it really bugs me when you talk to my boyfriend and maybe they will understand. Bring up an example and say you know you would not probably like it if I were talking to your boyfriend. Just remember to keep your cool and stay mature about it. The best thing you can do in this kind of situation is stay calm. If you get all worked up about it, she won't stop. So keep your cool.