Thanks, Paul. For all other prospective guests: there is normally a cleaning service that comes in and cleans between guests, which is covered by the cleaning fee. The previous guest before Paul asked that I waive the fee, in exchange for them cleaning the apartment themselves. Obviously, they didn't do a very good job, which is why I refunded Paul his cleaning fee.
It should be noted that this was the first time this particular guest has ever used Airbnb, and while I respect her concerns, the apartment was in immaculate condition when she arrived with the exception of the stovetop, which is very old and difficult to keep clean. I would encourage other potential guests to look at the photos and read feedback from other guests in order to form their own opinions.