What to write to someone that keeps adding you on facebook?

I think the "close friends and family" answer will work. Otherwise, I'd suggest that you add her under a limited profile so she can't really see anything about you and can't communicate with you. I would not talk about the ex-friend to her as it would only add to the drama. If they're friends, she won't be willing to hear you and she'll probably carry it back to your ex-friend.