I think my friend is trying to flirt with boyfriend

I think my friend is trying to flirt with my boyfriend.

I want to start this off by saying my boyfriend is not comfortable with what my friend may or may not be doing.

I feel like my friend is flirting with my boyfriend and I have no idea what to do. I've had the suspicious that she has been flirting with him for a while now because of things she's previously done. I thought it was over because she likes to claim she hates him and a bunch of other stuff but I think she's lying. She's liked him in the past and I'm really suspicious of her and I ultimately do not trust her. She is the really insecure type of person who talks to multiple guys to gain some sort of validation. I know she's got issues and I understand that but I don't feel like seeking male validation helps with that at all. my boyfriend brought up the possibility of her even getting more enjoyment out of it because we're friends and that really pisses me off. Any advice?