No decision should be made on the mocks results alone, for all the reasons you outline. Neither should it be ignored.
It is part of the process of deciding if you are prepared for the exam, and able for the honours paper.
Sorry if this sounds harsh, but if she has no other way of guaging if she is ready for honours maths than the mocks results, then she does not have a good teacher.
If she does have a good teacher, then she should be getting guidance on this issue,
She does need to have that mocks paper rechecked by another teacher or her own teacher, to get another opinion on whether the paper was harshly marked or not.
Maybe it was fairly marked and she made a mess of a question on the day, but is still an able honours student and can learn from the experience.
It is so hard on kids and hard on parents trying to help, but my advice is dont panic
and take these results for what they are and help her make a decision based on all
the information,