I should warn your friend that as the syllabuses have changed, sitting a chemistry a level in one year will be a different type of A-level exam to the ones that he/she sat in year 13 (I am presuming she has just left sixth form?)
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And from personal experience, going by the results of other people in previous years, doing a different subject (eg maths in one year, a couple of years ago) doesn't give a very good answer about whether this is possible. Plenty of people in my school sit maths AS, A2 and further maths in the same year and come out with As. Chemistry however is very different.
AS grades for chemistry have dropped for the first time in many many years. Edexcel in particular, marked down, rounding DOWN practical scores.
I go to an above average state school yet only 4 out of 60 chemstry students achieved As, whereas normally, 20-30% do. I myself, a straight A student, was so relieved to have scraped a C, when I was sure I had done so much worse.
It's a lot harder to do well in now, although your friend has probably taken the old syllabus science GCSE, meaning that it may be easier for them. If however they took the 21st century science- I would strongly STRONGLY advise against doing chemistry AS and A2 in one year, even disregarding the fact that the A2 course mostly relies on information studied in the AS course.
Hope that helps