Hi, OP -
With the PG of B the question is whether DD is enjoying FM? What she is getting out of it isn’t hugely likely to facilitate a Maths degree.
I hasten to add that I don’t see a red flag here. Often it’s just a question of priorities and intellectual maturity. But FM is more similar to degree level Maths than A level Maths is, so not liking it would be a reason to rethink.
Some Maths degree programmes, even very good Russell Group and similar ones, do not require (or strongly recommend) FM. The ones that do tend to be looking for, and more importantly teaching to, those with an A. For the sake of getting a good foundation which is crucial in Maths, DD might do best on a programme where most do not have FM.
Then the question is, although we now assume she likes FM if she is going ahead, are her interests best served by continuing? As a former Russell Group STEM admissions tutor - in a Maths adjacent School where most have at least grade A in FM - I would find PGs of three A stars more impressive than a fourth which was a B in FM, for any STEM degree. And it seems easier on the candidate
I hope this message sounds fundamentally positive, because your DD is in a very strong position overall. It’s just that for whatever reason FM isn’t really sticking right now, and in order for her to thrive as a UG a Teaching and Learning approach that caters for this will help her immensely.