Not being shown the paper is weird, how are they supposed to improve if they can't see where they went wrong? (Are they perhaps being held back until all stragglers have sat the exam?)
There is quite a big step up from Y12 maths content to Y13 content and quite a few struggle - was there any trig on the mock, that's a particularly normal area of issue.
With maths, she needs to be proactively spotting things that she doesn't understand and then taking it upon herself to work on them independently.
Useful websites: youtube.com/channel/UCyyRmnmtgVy5Sm7_UiCLFgQ is a channel of worked examples, particularly useful if she has the Edexcel textbook as it works through it, so it can ' top up' her lessons, but still good if doing another exam board.
alevelmathsrevision.com/maths-categorised-exam-questions/ has questions by topic, with answers and videos
As does www.mathsgenie.co.uk/newalevel.html
So there are loads of resources. What she needs to do is pick a topic, watch a video (but not passively, she needs to have pen and paper and actively try to do the question and unpause to see if she's correct), then try a question, mark it, fix it if she gets it wrong, try another question and do way more questions than she thinks is necessary, to make the methods automatic. A lot of students think that doing a couple of questions and getting them right is enough and it's not.