It sounds like its little neck was broke... How sick. Sorry to say this but getting her a new cat or trying to convince her it was an accident will not help at all... To her this represents all her accusations of your friends being a bunch of drunk idiots.
So even though it was an accident you better be kissing some major ass and apologizing for getting out of control. Don't blame it on the alcohol either. The alcohol while sitting in the bottle didn't do anything to her cat...
Man you're in deep deep shit. Her parents are going to think of this everytime they hear your name or look at you.