Not sure where all these "must occupy the first day" comments are coming from, the OP clearly said the renter "drove to the office, paid for the site...", so they were there the first day - it was not a reservation.
OP claims it was vacant the whole time, but was it really? Was the OP there, constantly checking? Or was the site possibly used by a motorhome or truck camper on an arrive late/leave early schedule, so it simply wasn't obvious that the site was being used?
Did the park allow one to reserve weekends on short notice (Tuesday before), or was the renter using the only method available to them to guarantee a weekend spot? Did they come in, then have some issue which forced them to leave unexpectedly. OP can't know.
Someone took the effort to go there physically, pay for a spot following the rules, and is getting dissed by someone who has sour grapes because they didn't get as good a spot.