My advice to her would be to relax and take the pressure off herself. It's not "do or die". It's an opportunity: the interview is the opportunity for both the tutors and for her to find out if the way she engages with her subject material are a good fit. It doesn't make her less of an academic, of a student, or as a person if she does not get in, or if she does and decides not to accept it: it just means that her learning style may be better suited to another institution. What you have in interview for many subjects you essentially have that twice a week for twice as long throughout your whole degree, alongside 1-2 other people. If it turns out that she passionately hates the experience, or cannot present her thoughts with as much clarity as the other applicants (who she would be with in those tutorials), it, she may be happier at a different institution anyway.
Many school mock interviews bear absolutely no resemblance to the real process, so don't let her be put off. I remember mine was someone grilling me for 20 minutes about why I was not taking a gap year because he believed everyone should take one, it was utterly and completely pointless. If the mock was so bad, just let it go, it's irrelevant and the interviewer was probably a jerk if he continued if she was in a state. The time would have been better used helping her to think about how she was going to deal with those nerves in the real thing.
The most important thing now is that she finds a way to calm and centre herself so that she is not so outwardly nervous. Think with her about how she will manage her time leading up to the interview. Does she prefer to be around people and distracted, or on her own? I remember one of the people I interviewed with would be sat in the quad in interview outfit nervously from the start of the day, even if the interview wasn't until 4pm because they emotionally needed to be there and ready. I on the other hand didn't start getting ready until about an hour and a half before as I wanted to be clean and fresh, I wanted my outfit to be as uncrumpled as possible, and because that actual getting "suited and booted" was part of my preparation process (this does not work if you then get pooled by your college, and may get sent for a third interview at very short notice, FYI).
On that subject, know that getting pooled is not a bad thing - colleges will pool people who they eventually offer places to as well, it is part of the process of ensuring that there is some consistency in quality of offer-recipients between colleges. From my own experience, they will pool people they are saying yes to, they are saying no to, and who they think deserve a place but they don't have enough places, or they simply aren't sure about, possibly because of inconsistent performances across the two interviews. The College may not have the full 3, 5, 8 or whatever places that there are in each annual intake for that subject, as there may be people who are holding those places on a deferred offer from the year before, or even people who have to repeat the year. It's a mysterious process but it's done with the intention of trying to make sure that some colleges are not rejecting several candidates who would have got in if they'd interviewed at the college next door because one has a particularly strong or weak intake.
In some subjects, they will get some material e.g. 30 minutes before and go to a room on their own to prepare it. If that's her subject, think with her about how will use that time - I suggest 2-3 minutes just to calm self down in the room - 2 minutes initial reading to see what it is about, 5 minutes deep re-reading and thinking, 10-15 minutes analyzing the material, thinking about what may be asked, making some notes, and leave 5 minutes at the end to have some sips of water, breathe deeply, adjust any part of outfit or hair that is annoying, repeat confidence building affirmations and mantras (I deserve this opportunity; nerves are just excitement because I am happy and excited to have this chance; I am a great person and they would be lucky to have me; I am a strong candidate and I am confident in my abilities), and just generally remember what her own name is, try to smile, etc...
Finally, from me to her - enjoy the experience. Oxford and Cambridge at Christmas time are magical places. Meet other candidates and have fun, make it a positive experience irrespective of the eventual outcome. The fact that someone is loving the experience of being there will shine through at interview an show that they will integrate into college life. The other candidates are not just "competition" - if she gets in, some of them will be friends, tutorial-buddies, housemates, the person she rescues from a dodgy bloke in a cheesy nightclub at 2am and the person who panic-prints her essay for 2 minutes before she has to leave for a tutorial because she has run out of printer credit. It is amazing and energizing to have a couple of days with a group of talented, interesting, bright, academic, high-achieving and high-aspiring peers with only a couple of hours in the day where you have something you actually have to do. It's tremendous fun if can allow it to be.
I wish her - and all your other hopeful DCs - all the best.