Why does my friend flirt with men who show interest in me?

We're in our early 30's and have been friends for 6 yrs. Any time we're out and men are around and start up a conversation with me she either starts telling them things about me that aren't very nice, like something stupid about how my hair looks. Or if after talking to a guy and I walk away, she'll come up to the guy and start up a conversation with them or start flirting. It makes me soo angry and hurt that someone I consider a best friend would do this. AND she has a boyfriend. It happens pretty often and a lot of times I didn't bring it up to her, but this last time she started talking to a guy that I had just walked away from. I watched her do the whole thing. I was so angry that the next morning I said something to her. She denied it and lied about what actually happened, said I was rude to say such things and I should have gotten the facts right. I was right there, 10 ft away from her and saw her walk up to him and start talking to him and flirting. Even asking him what him and I were talking about. Now she's making it like I'm the bad friend for accusing her of such a thing and that what I claim to see didn't happen. What would you do? Should i just drop her as a friend? Right now I don't want to see or talk to her. I have a huge problem w/ friends who lie and tired of her shady ways of hitting on sabotaging things for me w/ men. Any thoughts, or anyone have a similar experience, what did you do?