Was it a full mock- all 3 exams, or just one paper? If it was just one paper, which exam board and was it paper 1, 2 or 3?
Which year's grade boundaries were used?
Did she manage to complete the full paper in the time allowed?
What did she do when she was struggling with a question?
Does she have any thoughts as to why she did badly?
I would say our Y13s across the sciences are finding things tough- I think having never sat formal exams (even though they've sat lots of assessments and a set of Y12 exams) has impacted them. But I wouldn't say they are all getting 2 grades below their target (is the B a realistic target, or was it always an optimistic prediction?).
Saying "the whole class did badly" isn't always very meaningful- it's likely to be a group of <15 students and there can be big fluctuations in ability year on year. And it also won't necessarily help her achieve her uni offer.
I do think grade boundaries will be low(ish) this year, but I think it might also be worth speaking to the teacher and asking for their take.
Is there any possibility of getting a tutor to help with a bit of last minute cramming over Easter?