How do i ask my friend to stop flirting with my boyfriend?

Okay so long story short, my botfriend has this girl that hes been friends with for years and im friends with her now. She always hugs and leans against him and tries to hold his hand, is constantly being verbally flirty with him and one time she literally grabbed his hand and kissed it which isnt really a huge deal because i trust him completely but it still makes both me and him uncomfortable. hes tried asking her not to do that kind of thing anymore but she continues to. She used to apologize to me and say that it was because she wanted to make her ex jealous but her ex isnt even around anymore and she still does this. she just has no concept of boundaries, and i love her but damn. how can i bring this up to her without making her feel hurt or like im trying to cut him off from his friends?