What's to stop me as a grown woman deciding to study GCSEs and a levels I didn't choose to study at school?
Nothing, if you have the money and if you need it, can find a tutor. It's not slways cheap.
I have done German and Spanish GCSEs and AS-levels at evening classes - I'd have done A2 as well, were it possible, but not enough of us wanted to do it for them to run classes. I wanted a taught class for the speaking as much as anything, as I'm fully aware my accent is rubbish. I'm currently learning Welsh, though probably going to opt not to take an exam at this point of a number of reasons, including availability.
It's a bit odd taking exams with a load of teenagers, but it happened even in my day - we had a mature student join our French A-level class back in the '80s. I quite enjoy a good exam, now it doesn't mean the difference between whether I can leave home or not. There's something about the challenge of answering a question within a set time that I enjoy. (Not thst all subjects/questions are essay-based.)
I've also done an OU module for fun. There are tons of free courses online these days from all sorts of providers. If you're in a university town, it's worth checking if they do any extra-mural classes. There are also organisations like the WEA, even if your local authority doesn't have much in the way of AdEd these days.
I have always thought we narrow education too early in England and Wales - I still resent that I could only take 3 subjects at 6th form. I'd have been much more suited to a bac-style system.
I hope she's building up her hand strength - even when I did writing by hand every day, two three hour exams in a day would have my hand aching and starting to cramp by the end of a paper. Although a friend who teaches says they do get their boys to do hand exercises in preparation.