I read history at Oxford and am now a solicitor, I knew I wanted to do law as a career so thought I would take the opportunity to do something else first! Oxford was certainly a fantastic place to study and having an oxbridge degree is the key, ime, rather than what that degree is in iyswim.
If she is interested in studying foreign languages then that might be a good option to study as law firms she might apply to in the future generally consider languages an added benefit.
She should consider whether she wants to be a solicitor or a barrister. The majority of my colleagues did other degrees first and then did the law conversion course (Postgraduate Diploma in Law, gets you to the same position as having done a first degree in law). However ime most barristers have done law degrees as they generally need a greater understanding of theory than solicitors.
From friends who did law at Oxford it is quite an old fashioned degree (lots of Roman law etc) and therefore not ideally suited to practice.
All the ideas mentioned above are worth doing, also candidates for law at Oxford now have to do an entrance test, see if you can get hold of a couple of sample papers for her to try. Generally the interviewers, ime, will be looking for someone who can "think outside the box" cliche though that is. So encourage your dd to read widely, question everything and show a real enthusiasum for her chosen subject.
Oxford do offer a 4yr law course with a year spent abroad studying european law, however it is extremely competitive and only exceptional candidates are likely to get through.
The other thing she should consider carefully is which college she applies to as the tutors/atmosphere/facilities vary considerably (even though they tell you they don't). It is worth going and checking out the colleges that she is interested in, attending their open days and speaking to the subject tutors as they are the ones who will be interviewing her so making an early good impression can be helpful. Also once she knows who the interviewers will be she should research them, find out their interests etc so as to be as prepared as possible.
Sorry this has turned into rather a long post! Got carried away, loved my Oxford days....