Another academic here who agrees with @flyswimmer.
Basic arithmetic shows that a 2.1 is still easily achievable - on paper, that is. Without contradicting my certain knowledge that one can scale career heights following a 2.2 (I once helped a desolate tutee compile examples) it is undoubtedly true that life is much easier with a 2.1
It is good that DD knows her problem is with maths. The question is, how is she reacting to this? Many to whom school and perhaps Y1 came very easily come a bit undone when they first struggle. Although they know this is a fallacy and would never admit it, they may feel some shame that the material now requires effort. The vibe of effortless cool certain Oxbridge students cultivate can make this worse. (This is usually phoney or else they are poor students but their impressionable peers don’t see it that way. Only a few can do Oxbridge Economics easily, particularly at C)
With long experience in a maths orientated STEM subject I believe DD can get a 2.1, but that is irrelevant.
She needs to work through her inner obstacles and master the Y2 prerequisites for her Y3 modules, so she can go into them with confidence. Then she needs to engage properly, attending all lectures as well as tutorials, processing them, making a really good effort week by week, acting on feedback and getting difficulties cleared up promptly. This may mean a level of engagement with her tutor(s) that she is not used to, but that tutors are more than fine with as long as tutees are using their time efficiently and working hard.
I rather suspect that proper revision of Y2 and proper, not extraordinary, engagement next year will do the trick. This is not the typical approach by any means nowadays.
Best wishes to DD