Solitude my DS is taking the 4 A Levels you mentioned.
Essentially what we've been told by the school is as follows:
Taking only 3 is fine and what most Uni's accept.
However, if you're thinking of doing maths at Uni the doing FM is an advantage as it's pushing into the degree syllabus so gives you a bit of a head start.
As a pp said there's also a cross over with Physics, which again if this is a potential degree option can be helpful.
All that said, the drop out rate for FM is the highest of any A Level. Mainly for 2 reasons.
Firstly it's a lot harder than "just" maths and even very competent mathematicians can find it a slog (which is why most schools want an 8 as a minimum and even then some students struggle).
Secondly, because it's "not strictly needed" if students find themselves struggling with 4 A Levels (or with the step up to FM) it's the obvious choice to drop to focus on the others.
In my case maths is DS's best subject (or rather passion) and the idea he wouldn't do FM is an anathema. He's already read the A Level Maths and Further Maths revision guides over the summer (and is current on Physics). He got 9's in those subjects - they are his strongest, save Chemistry (also a 9) but he's less interested in that.
The school have strongly supported him doing FM not just because he's very good at the subject, but because he really, really enjoys it and apparently is a "gifted mathematician".
So wrt your DD I think it's worth considering how much she really enjoys maths and does she really need to do it (is she planning on maths/physics at uni)?
Is she also a natural mathematician? I know she got a great grade, but again we were told by school that great grades at GCSE can be gained and though hard work but FM really needs some good raw ability as well as intense study (which is why even students who did well at GCSE still drop FM).
On one hand there's no reason she can't take it and drop it should it not work out, but I'd consider is she a child that would do that philosophically? Or is she someone that would struggle on regardless to the detriment of her other subjects, or perhaps have her confidence knocked if she feels she needs to drop it?
As such I think it's only a decision you can make together perhaps with some input from the school's maths department, but overall if she's keen, loves maths, thinking of doing maths/physics at uni and wouldn't be phased by dropping it if needed then it makes sense to give it a go imho.