I think you perhaps mean taking holidays not holiday accrual.
I suspect that is partly what is causing some of the confusion in this thread - that an employee can request holidays (i.e. paid time off) during sick leave.
Requesting holidays doesn't mean you're necessarily going on holiday (you could simply decide to use those paid vacation days*) and likewise going overseas for a break can be done on sick pay (depending on circumstances), doesn't necessarily need to be taken as holiday.
*I took holiday to have knee surgery once, I didn't want additional paid time off as it would affect my profitability and therefore my variable pay
Nope, not at all. The important points here are the reasons he's off work and that the doctor has explicitly recommended going!
His reasons for being off work certainly don't prohibit him from going to Butlins and in fact, going to Butlins might well help with aiding recovery from the reasons he's off work. Whether it was pre-booked or not is completely irrelevant.