If she can do what has been taught so far (as shown by the fact she has got an A), there is nothing to suggest she will be unable to cope with what she is taught at uni.
Not everyone reads around the subject at A-level OP, most people learn what they need to to pass exams, which she clearly has judging by her A grade.
Also a lot of people will be doing maths at uni having got below an A in A-level maths and cope just fine, so why wouldn't she?
Finally a love for the subject cannot be underestimated. It will help a lot at uni.
No offence but the fact you advised her to not do maths despite the fact she got an A at A-level suggests you are one of those dicks who puts people down when it is clearly unwarranted. 'You aren't cut out for maths. You might get an A, but you aren't good enough as you got that question there wrong, which was an easy question - seriously how could you get that wrong? You clearly aren't good at maths hahaha'. Basically destroying her confidence when there is no need to do so as she is doing more than fine.
Edit: just as I suspected, you got high UMS at AS. I.e. one of those people who are stuck up and look down on people who do worse than them and say they aren't good enough to do things. Not a good attitude to take OP. I did well at A-level and don't rub it in people's faces - you make no friends doing that and I am surprised your friend didn't react in anger to what you said.
If an A is not good enough to do a subject at uni, then what is? Not everyone can do as well as you at A-level OP.