Your best friend should be someone you can trust completely, and if she's flirting with your boyfriend, it can feel like a betrayal. Before you jump into a confrontation, look closely at what's going on. If your friend is naturally flirty and treats your boyfriend the same way she treats every other guy, her behavior is probably unintentional and not cause for concern. On the other hand, if she's definitely giving him special attention, it may be time for you two to have a serious conversation.
Set aside time to talk to your friend when your boyfriend isn't there. Don't wait for her to flirt with him in front of you and then pull her aside for a confrontation; that can introduce drama and conflict into a relationship you're hoping to heal. Instead, go out for coffee together, or plan a girls' night in. Choose a setting where you can talk for as long as you need to.
Talk to her frankly and directly about the concerns you have. Don't beat around the bush or try to lead her into bringing up your boyfriend herself. Instead, say something like, "I need to talk to you about the way you interact with my boyfriend. Some of your behavior makes me uncomfortable."
Give her specific examples of things she's doing that are too flirtatious. If she's not aware that she's being overly friendly -- or if she isn't the friend you think she is and she's doing it on purpose -- she might deny that she's flirting. With examples of things she's done or said, you can demonstrate that you're not imagining it. Keep in mind that if you can't think of any specific examples, you may be imagining it after all.
Speak calmly, without anger. Talk about how you feel and your specific examples, not about your friend as a person. This helps avoid hurt feelings and makes it easier for both of you to be clear-headed.
Listen to your friend's side of the story. She might not realize how forward she is with him, or she might have felt safe flirting because she knows he's taken. Staying open to conversation can help you avoid a painful confrontation and make it easier for you to understand what's really going on.
Ask her to stop. She may be sincerely apologetic and promise not to do it anymore. If not, think seriously about distancing yourself from her. Even if she doesn't think that what she's doing is flirting, she should respect your feelings enough to change her behavior; refusing to stop flirting with your boyfriend shows that she doesn't respect you or care about you enough to deserve your trust.