From what you say, it doesn't sound as if he noticed any of it and is oblivious. Some people really are blind! Maybe his wife noticed, but didn't want to mention it to you and cause him embarassment in a situation that was already far from ideal. So I wouldn't mention it either if he asks to rebook. You say they were a lovely couple, so if all else was good with the rental, then charge full price or even hike the rate $100 if possible to cover the loss (if you don't publish rates online).
I don't think you have to go running around getting him receipts for his treatment, this is well beyond the call of duty, but given the exceptional circumstances that he went to an emergency dentist, I don't consider the towel damage as negligence, disrespect, or assume that there would be a repeat. More likely, it's a case of "the price of doing business" and all that.
On the other hand, I can't imagine any treatment by a dentist that could have caused damage to 6 towels and a duvet. Are you sure this was the cause and not something else? To be on the safe side, I would probably substitute old towels next time around. If they complain the towles are not as good as last time, you can give them the reason at that time, making a joke of it even, but I don't think this is something that you want to confront him with head on in advance of another booking. It's only going to cause him embarassment and you may lose the booking from making a "big deal" out of it. Better to hide the costs in the new charges and make sure you check the place thoroughly before releasing deposit funds next time.
Just my 2 cents...
