My Bestfriend is flirting with the guy i'm hooking up with, what do i do?

So this could be a really long story but i'm going to shorten it up. My bestfriend who has a serious boyfriend, has been hardcore flirting, texting, and has been planning to hangout with the guy i'm hooking up with. She tells my friends that she doesnt care if it upsets me and that she is going to do what she wants. This is so upsetting to me, I know she is in the wrong especially since she has a boyfriend. I don't know what to do, do i talk to her or do I talk to the guy who i'm hooking up with? and a sidenote she isn't very open minded to listening to people, so I feel like talking to her would be a waste.