Ian, Does the school she goes to, or do you know anyone, who have been to Oxbridge themselves? My school organised mock interviews and for Oxbridge applicants, one of the parents (who studied economics at Cam) to do the interview.
I would not recommend that it be anyone well-known to her. She will not know the tutors at Wadham, they will not know her, and one of the hurdles in an interview is to be able to respond well and almost warmly to people that you've never met before.
you may wish to give her some questions and that's a very good idea. One of the best ways of preparing is to think about, and structure answers to possible questions. They are very likely to challenge her points at interview - she should think about what sort of criticisms they might bring up and her response. It's a very good way of consolidating and re-evaluating your own convictions.
If she's put any cases/legal issues/books on her PS, then she should go through these in detail, get the gist, pick up on what she agrees/disagrees with, and be able to say why with clarity. Even if it never comes up, its worthwhile to go through it (just in case it does).
Formal interview training might help but i would imagine it tends to be too generic. It will offer ideas about what you should do, how you sit, how you present yourself etc. Oxford interviews (in my experience) are academic. In both of mine there was no focus on any extra-curricular activities, anything in my PS, anything about what I enjoyed in the subject. Therefore, despite the fact that you've said she interviews very well, she cannot rely on bringing herself across as a bright, interesting person. They are looking at her academic ability and academic potential first and foremost. A good interview technique is highly unlikely to balance out a weakness in this area. She would, imho, be better off doing a few mock law interviews and spending the time thinking about the subject she wishes to study for 3 years, rather than going to interview training.