First, why didn't the guest tell while they was there? Recently I had a guest complain about the internet in his review, There was a problem with it had he have told me during his stay I could have fixed it,
Why offer refunds? Let them bring the subject up first, people do understand things can go wrong as long as you are seen to be doing something about it,
I just remembered 2 guys rented a villa with us and they did the same told me afterwards they had no hot water in the shower, it was only a fuse which was fixed as soon as I knew about it,