Do you mind me asking why you think she needs paid-for preparation?
If she has had unexpected issues (illness; teaching significantly worse than that you'd expect for her peer group), it'd be better to explain it in her statement (and everyone knows this year is going to be odd because of covid).
Often, IME, candidates who have been 'prepared' for interview are a bit rigid. There are some companies out there that are, basically, scammers, who will drill your child in techniques and learned phrases that they promise will help. The actual effect is often to make the candidate panic in interview, because there are no techniques and 'correct' phrases, and interviewers will tend to push at anyone who seems to be working from a script.
If it's more the sort of service where someone knowledgeable will chat to your DD about her subject, that might work.
I am trying to be even-handed writing this, because I really do know that sometimes candidates who might be excellent, struggle in interview because they feel very shy, or they've not had much experience talking about their subject. And it absolutely can be an issue of educational/social privilege, and I would never want to pretend otherwise with some pollyanna 'oh OP, don't worry, Oxbridge interviewers are all psychic and never get it wrong'.
But it is also really frustrating - and sad - when you interview someone and they have been taught that the interview is about this strategy they've learned. Or they expect certain questions (often very pretentious or unlikely questions) and are stumped by the questions they actually get.