I would say definitely visit places before applying. I was put off some places for law based on the information I received on open days.
The first decision she will likely need to make is whether she wants to do "straight law" or whether she wants to do Law with French Law. Oxford has this as an option on the Jurisprudence course (not sure about Cambridge, but suspect it does on the Tripos for law also....). There are fewer places for the 4 year course, but certainly at Oxford they interview together and if they think you are worthy of a place but were edged out for the year abroad course by others they will still offer a straight law place. I know people who did the 4 year course with Y3 in France, and had an amazing time on the year in Paris. It gave them a bit of respite from the Oxford bubble and to digest everything from Y2 a bit better.
The Oxford course is exams all at the end. It is highly stressful, it is one of the worst experiences I've gone through - 9x 3 hour exams in 10 days is absolutely insane. But it is excellent preparation for the Bar, which is why a lot of Chambers look favourably on pupillage candidates who have gone through it and come out with a decent degree at the other end and haven't crumbled under the pressure - it's the ultimate work-memory-resilience-pressure test. I didn't enjoy it, but I haven't regretted it either. It's taken me a long time to get rose-tinted glasses about Oxford, but I am there now. I wouldn't change it, but if I had my time again I might have done a history degree and then the GDL to have a more chilled out university experience - the jurisprudence course is intense, and the lawyers work very hard compared to some other subjects, it is relentless and rigorous.
The other difference between the Oxford course and other courses is that the amount of choice is very limited. You only get a couple of elective subjects, and in additional to the normal qualifying law degree topics you end up studying random additional mandatory subjects - Roman Law, Jurisprudence, Public Law is split into Constitutional Law in Y1 and Administrative Law in Y2. If they are things she might enjoy, great. They aren't known for being the most useful subjects in actual legal practice - they are designed more to be about the jurisprudence, hence the course title - it's about the theory and academic study of law. I was told day 1 that "We aren't teaching you to slum it in the magistrates' court, we're preparing you to argue in the Court of Appeal", and that was fairly accurate to be honest!
In terms of the LNAT, assuming it's the same as when I did it, absolutely prepare. Understanding how the questions work and how to answer them is really important. But don't be put off by the practice papers either. I did terribly on many of the practice papers and was completely freaking out, and ended up with a really high LNAT score, and I'm sure it really was all the practice and learning to understand how the questions worked coming good on the day.