The maths required for ALL students is GCSE. The rules now are that anyone with a D at English or maths MUST continue with until they achieve a C grade, or turn 19.
Functional Skills, and similar, are acceptable as a stepping stone towards that C grade at GCSE.
OP, she is probably utterly defeated by maths. I taught many who were. They were so bewildered by the teaching or the exam that they were convinced they would never ever pass. Stats seem to bear this out - far less than 10% of 16 - 19 year olds who retake maths gain that C grade.
I haven't seen stats for last year in FE (they are quite well hidden) but I do know that in 2 very good/outstanding local colleges the GCSE maths pass rate was less than 2%. I suspect that this will not be unusual nationally.
Basically the enforcement of GCSE is simply not suitable for the vast majority of students it is pushed upon. Imagine, they have had 5 years of hot housing by schools, 9 hours over 2 weeks, extra sessions etc. Now they are in FE, where there quite simply is neither the expertise, support nor time in the timetable to match the input in schools. So if, despite having been identified as a possible D grade achiever and hot housed by schools they do not get a C how will they achieve it in FE, given that their focus will be some other much more interesting subject/course?
If you can get her to open up ask her to tell you why she is so adament/scared. You may be horrified at her answer.