Here's what I would do. Show empathy, say unfortunately costs already incurred including cleaning but you would be happy to offer them another night for free if they book at least 1 night another time....And the dates have to be agreed up front. An extra night for me in addition to a booking doesn't incur any additional cost especially if its on a quiet day.
Hotels do this even if there are issues with the room stay, they won't refund, but they'll upgrade or provide a 'credit' for another night as long as you book another night with it.
Hope this helps
Regs
Mary