I (20F) am uncomfortable with my friend (22F) hanging out alone with my boyfriend (23M) and i don’t know how to tell her

As i said in the title, i feel uncomfortable with my friend hanging out alone with my boyfriend, but i don’t know how to tell her. I’ve known her for many years, and i don’t think she has bad intentions, just often times can’t control herself. She knew him before i did and i met him through her. The reason i don’t fully feel comfortable is because i know she has a personality that can be seen as flirty. Once when she was drunk she kissed my boyfriends neck, and he told me about it like the day after. He also didn’t tell her to get off of him or anything, which did upset me a bit, but i know that it’s because he’s just kind of scared to stand up for himself, even when people wrong him he often doesn’t do anything about it. I’ve already talked to my boyfriend about this and he’s fine with not hanging out with her alone, but now we don’t know what to tell her when she asks to hang out with him. Should i tell her that i’m uncomfortable at all? And if so, how do i go about it? I don’t want to offend or hurt her in any way cause i really don’t think she does it maliciously or on purpose. So, how do i go about telling her?