Sorry to say this, but nicely won't do it. Not in the way that you imagine it anyway. You can cut him out entirely while being nice about it, but you can't keep him as a friend without giving him a very strong talking to and creating some distance.
You've already rejected him nicely and he's just digging in deeper. He will continue to do so until a drastic change is enforced.
I'm assuming at some point she's going to calm down and act more adult like?
Crazy only gets worse, it never gets better.
Sorry to say this, but nicely won't do it. Not in the way that you imagine it anyway. You can cut him out entirely while being nice about it, but you can't keep him as a friend without giving him a very strong talking to and creating some distance.
You've already rejected him nicely and he's just digging in deeper. He will continue to do so until a drastic change is enforced.