I think I would say they would have to be prepared to loose their deposit, would have been paid prior to all this Covid 19 stuff so would be covered by insurance. You will have already paid out from this if booked through this site for referral fees.
My end of May guests cancelled prior to paying balance as the dad worked in the forces and leave had already been cancelled by then.
My end of April (US guests) have moved to August dates.
If travelled was allowed at end of May I would let them pay up to a couple of days before