If she'd be unhappy if she went anywhere except Oxbridge then she's setting herself up for a crap 5 years or so. Nobody has a good chance of getting into Oxbridge, so basing whether or not you'll be happy on it isn't a good way to go about things.
And I know that her chances are going to be slim (slim, not non-existant) because A) Oxbridge is notoriously difficult to get into for even the best of applicants, and B) They'll have more than enough applicants who have double the number of As and A*s at GCSE than her, as well as the 4 sciences at AS (and probably other subjects too). And apparantly Oxbridge prefer sciency people. Those are facts - and she'll need to take that into account when applying there.
Like I said, there's always the chance the OP will get an offer. But to do so, she'll have to pull an absolute blinder on the BMAT, get straight As at AS and A level, have an unbeatable PS and reference, as well as a whole lot of luck on top. Basically, if you're applying there, be sure that your other 3 choices are way easier to get into or you're asking for an unwanted gap year.
Don't just apply there blindly because it's your dream, think carefully about it first.