I can understand if the petsitter also has a day job. I petsit as well as my day job of grooming. I would suggest they find another sitter than can help out with an afternoon visit for some play and potty time. I'm lucky enough that majority of my clients live very close to my work so I take my lunch with me to check on furbabies. The only other thing if you can't find a 2nd helper would be to board them at a kennel. I know most people don't like doing that though You may try calling local vets, they probably know of someone who will do an afternoon visit.
Good luck!