I'm on the fence here.
My son graduated from Oxford in History last year and had a fantastic time. He did work very hard but had plenty of time for socialising and other interests too. Having said that, he copes very well with pressure, is excellent at time management and has (thank god) not inherited the procrastination gene from his mother.
If any of those had not applied, I can see it would have been a more mixed experience.
In his case, belonging to a college was a huge plus point. It seems to have made it a lot easier to get to know people and make friends. His college was very friendly and had a good mix of students.The pastoral support was very good - this may vary from college to college, but his college seemed to have a very good system. The accommodation was fine and the costs of studying a lot less than many other universities (short terms, subsidised food, no travel costs while in Oxford because everything was walking distance, accomm. costs lower than average for universities, fabulous library provision, which meant he scarcely had to buy any books).
He also got on well with the tutorial system, which is absolutely crucial - if you don't like the idea of weekly tutorials with only one or two other students, Oxford and Cambridge are not the places for you!
On the other hand, I had an offer from Cambridge a million years ago and would have gone there if I'd got the A level grades. I am so glad I didn't, though! My insurance offer was 2 Es from UCL (those were the days! late 70s, wouldn't happen now) and I loved, loved, loved UCL from the moment I stepped through the gates. I distinctly remember thinking that if I hadn't applied to Cambridge I would definitely have accepted UCL but I should stick with Cambridge because it was, well, Cambridge. I did like it and I'm sure I'd have enjoyed going there, but I hadn't fallen in love with it as I did UCL.
So it all turned out for the best for me because I did end up at UCL and I was very, very happy there - the course was great and I liked living in London that I've never left. I think I might have found living in a small university town a bit claustrophobic.
Good luck with the decision! She could always firm Bristol and have Oxford as her insurance - a luxurious position to be in!