I agree with Limeslice and Divvy. But don't get too beswipst before calling.

Or you could always text her from your DF's phone, with his knowledge. Or respond to facebook with "and you really think I'd want you as a bridesmaid after that? That's right, I don't." (Actually, just un-friend her on facebook and save your sanity.)

As you didn't ask her, you could always just not get her a dress/outfit, not tell her when the rehersals are and act all surprised when she thinks she's a bridesmaid. My mum asked some of my cousins to be bridesmaids/pageboys that I didn't want at all, so I ignored the whole situation and she eventually had to un-ask them because I wasn't doing anything to include them. Wow, I'm a wuss.