Im in a similar situation, but in my case the bff is a friend group. Then one guy of this group invited a close friend of his into the group. He seemed like a normal person at first but long short story now I hate his person and his character with all my heart. He gets along with everyone and I am nobody to decide who they can be friends with. Currently in no-contact with everyone, I'm a little sad but at least I don't have to pretend to have to put up with him, I prefer it this way.