@Shannon15 Hi Shannon, these days I will usually try to answer questions in the form of a message to the person posting but the message option is not coming up on your profile so on this occasion I will offer a public response.
You have the word of your cleaning lady, your mother and a friend (all of whom wittnessed the apartment prior to this guests arrival) that the apartment was in a fit state to let, and none of your previous reviews suggest that cleanliness is an issue with your property.
I may be wrong but of late this seems to have become something of a sport on Airbnb and possibly other hosting sites......Invent a problem and get your money back!! And these days Airbnb are more reluctant to offend a guest than a host!
Shannon, stick to your guns and don't refund what this guest is asking. They may wish to file a claim with the resolution centre, which they is entitled to do, so as soon as you can inform Airbnb of this incident, keep all your documentary evidence including possible photos (dated for preference) as you may need to defend yourself.
The problem with giving in to this guests demands is, in some instances it makes them braver each time it works! I have seen profiles where the same guest has used this scenario on other occasions! Maybe they are particularly fussy, but it is a red flag to me.
I understand you don't want to jeopardise your superhost status but if you can prove to Airbnb that the guest has fabricated a complaint they WILL remove the offending review. And if it stands Shannon, in general the public are not fools. One bad review in a heap of good ones sort of stands out like a sore thumb and is treated as what it is.....sour grapes!
Good luck and keep your standards up....cheers....Rob