“Radical acceptance” doesn’t mean you accept or allow bad behavior. You accept the fact that this person is a mess, it can’t be changed, they are who they are and you just not going to be influenced by them. Their opinion doesn’t matter

You distance yourself (emotionally or physically) or break up or stop friendship or whatever you need to do but you get them out of your head. You don’t spend your life ruminating why they they say this or that. Accept that you can’t control them. You can only control yourself. Radical acceptance isn’t being a doormat. It’s a bit opposite actually.