I'm sorry, but dump her ass. Personally, I've never been in that situation (thankfully) so I don't know how I'd really react, but just hearing about it makes my blood boil.

Even if she did it when you were starting to go out, that's no excuse. You wanna fuck someone else, at least have the decency to tell the peson you're with that you're interested in someone else. That's the way I seee it.