The only issue is the legal one. Practically it makes total sense to do the work and save your job.
The legal considerations seem to me to be between the Govt and the employer, not the Govt and the employee.
I appreciate that you have far more understanding of this situation than me - I took my info from https://www.wired.co.uk/article/uk-furlough-scheme-job-protection
I also appreciate that if the employer breaks the rules, is caught out doing so, and has to pay the money back to the Govt then the company may still fold.
In which case, it still seems like a no-brainer to me - work a little and give the chance for the company to keep going, vs work a little and risk the company not receiving the 80% funding (which will likely have the same end-result as not doing the 4-6hrs requested).
I really appreciate that we are into very murky waters morally, but I was trying to address the matter simply from the OPs son-in-law's perspective.
Anyway, just sharing an opinion, which is what the OP requested, not trying to state an absolute right or wrong here.