@Lee336
Lee I am sorry you have been caught short by this guest. Some of them can have incredibly high perceptions and expectations of cleanliness. You do seem to have admitted that there was a failing on your part when these guests arrived, and I do think you are doing the right thing by agreeing to refund the guest.
However you are probably going to have to do a bit of grovelling here to try and avoid a bad review. Say to the guest."I am terribly sorry that I was not better prepared for you, it is not typical when you look at my other reviews, and this is a cross, a wake-up call I will have to bare. I hope that my actions to fix this will go some way towards righting the ship, and softening your disappointment".....they will at least think you are a nice guy and may be lenient in the review.
Bad luck Lee, mate, we all get caught out sooner or later.....I did, a guest came up to me and told me to sack my cleaner! His wife found a used childs nappy under the bed.....I felt like bloody 'Basil Fawlty'!
I did soothe them to the point where I still ended up with a 5 star.
All the best Lee.
Cheers.....Rob