Perhaps I'm out of the loop, but I think that calling a 13 year-old a "little bitch" is out of line.
It's not going to happen in my home again. I don't think it's one bit funny or cute. And I promise that I'm going to stand up for her if it ever happens again in my presence.
I defended her over and over these past few days. I complimented her. Told her that she was a perfect house guest. We sat on the bed upstairs chatting and laughing. She's a lovely teenager and caused not the first problem while she stayed with us.
I hope she doesn't know that her grandmother calls her that.