my friend is hiting on my boyfriend

Approach your boyfriend. Tell him this gal is being a jerk and trying to get between the two of you.

Why him?

Because your relationship is with him, and not her. You could conceivably talk to her, yes, and tell her to knock it off, but she has little to lose if she fails to change her ways. At least this way, you make him aware of the behavior and that it is a danger to your relationship, and give him the opportunity to make up his own mind.

He might, by the way, choose her, no matter what you do.

If he does, then shrug your shoulders and don't think another moment about someone who would choose this kind of person over you.

Tell her indirectly like " i know this girl hitting on my bf...." saying exactly what you know, then threaten some action. A direct confrontation can end up bad coz she will be defensive and your point wont get across. She will have food for thought.