Duigi, are you a maths teacher?
No, I'm a lecturer in a subject that requires A Level maths, so I've looked in detail at syllabuses, as well as having children doing A level maths.
What do you suggest? Should she resit?
If she's wanting to do A Level maths, then any study of FSMQ is worthwhile in isolation. If she understands the material, she'll cruise through C1 and have a head start on C2. However, if that's at the expense of time and effort spent on other GCSEs, then the FSMQ isn't the best way to use limited time.
The FSMQ as a qualification in its own terms is of limited value. If she's got to spend the time in the classroom, then she should work on the stuff she understands or thinks she can understand, and not worry too much about the stuff she just doesn't get. It'll presumably be properly taught next year.
And, just to be controversial, if I had a child with a GCSE A in maths taken early, who was sat in a classroom in which GCSE maths was being taught, I'd be tempted to have them retake GCSE maths to get an A*. She presumably hasn't done GCSE maths now for eighteen months and only got an A anyway: the thing that stops A Level students stone dead in their first month is apparently algebra, just being able to be completely reliable in manipulating terms, surds, factorising, etc, etc. Given the choice between 100% UMS in GCSE maths and scraping a C or D in FSMQ, it would be interesting to know which would provide a better basis for A Level.