If you are an A* student at gcse maths, then unless you literally do nothing the entire year, you will most likely get a good grade in AS maths. AS maths is fairly straightforward once you get the hang of it, just practice a bit on it every week and do a bunch of past papers towards exam period. A2 maths is where it gets rather tricky though.
If you're an A or high B grade student at GCSE maths, then you will need to put in quite a bit more work, but it should all come together nicely by the time your first AS maths exams starts.
Best of luck in your GCSEs pal!