The problem with GCSE maths is that it will not acurately predict natural mathematical abiity or talent, where as A Level Maths do need some level of natural mathematical talent.
The new 9-1 GCSE maths grade will be increasingly predict natural mathematical abiity with far more accuracy. However, it will be still difficult to predict how a student would fare A Level Maths.
I say natural talent, because students can be developed and trained to a significant extent for A level maths, and I think to reach independent, high level Pure (and/or Applied) Mathematical thought process, one has to be naturally talented.This is my opinion.
However, if they can do some sort of mind imagination for explaining mathematical concepts, the it may well be a good indicator.
So, why do not they get some AS level books and try by themselves?
Hope tis helps.