Noted. By all means try ADR but I'm not convinced that the considerations about weekend trip/length of stay are valid and can be taken into consideration. If your parents had stayed a week in London it would probably mean less if they had to depart 7 hours earlier, but is that a risk of the airline that your parents booked a short trip and it was reduced significantly by the 7 hours? Anyway good luck!

If it had me I would have considered asking for a rebooking of the cancelled flight to later/high season to say Spain (where the fares are much more expensive) and bough the new ticket on the 12:25 FR flight on my own (but then you obviously can't claim reimbursement of the KLM ticket). FR has a quite generous rebooking policy for cancelled flights.