The authors should check when the journal transfers copyright. As mentioned by Andrés E. Caicedo in the comments, some journals assume the authors transfer copyright once a paper is submitted, not after acceptance.
If the authors withdraw a paper because they do not want to transfer the copyright, the journal's likely reaction would be to remind the author about the possibility of publishing open access. If the authors decline that too, then the journal will probably think, "why did you submit it to us then?" Getting the paper reviewed involves a nontrivial amount of effort. If the journal decides the authors submitted the paper knowing that they will withdraw it if it's accepted, effectively wasting the journal's time, they might blacklist the authors.
As for what happens to the peer reviews, from the journal's perspective, nothing. The reviews are left on the manuscript's record, of course, but nothing beyond that. The authors will still have access to them, and they can do whatever they want with them (such as use it to improve their manuscript). They should not, however, present them to another journal. As I wrote in an answer to another question, these reviews don't help the new journal:
We can't use the original journal's reviewer comments & your responses. We don't know who the reviewers are. We can't tell if the reports are legitimate. We can't see if confidential comments were submitted.