There are only a few subjects that are not linear and count towards A level grade. If it is an old style A level where AS count to half the grade you can resit next Sumer (same time as this year) and sit A2 at the same time
That would be an internal school decision. The AS doesn't count towards A-level so resitting wouldn't make any difference to final A-level grade. However the school may be saying that if she doesn't achieve a good enough grade this year, then continuing next year would be a waste of a year as she would probably fail the A-level.