Yes, Herman is right, in the NVR he can't go back. It really is a rapid fire paper.
With the VR he can go back. There is a school of thought that, should a dc be running out of time then they should tick (for instance) all answers as being 'a' or 'b' (or c or d etc) as percentage wise (depending on how many questions are left of course) you're bound to get 3/4 right. THEN if time permits, he can go back.
He's done one paper. The next ones will be easier as he knows what to expect (of course, the questions might be harder though
Good luck to your sister's dc too!!