I have a friend like this (yes, I think of her as a friend--a looong story), and what I do is tell her I can't deal with this or don't want to talk about this with her. If she keeps talking about it, I say, " I'm hanging up in...3 ....." she's still talking? "2...." she's still talking? "1...." she's still talking? "BYE!..."...bang.

She never gets mad. She never calls right back to complain. She never mentions my hanging up. I think she gets it. She doesn't get any better but ... I think she gets it.