Karen's advice is correct.
Moving forward, however, I would handle it this way:
Some dogs are more prone to hot spots than others. The source of the hot spot may have had nothing to do with you or your home. Rover's insurance policy, however, will cover vet expenses for you if you need to take a dog to the vet while the owner is away. The owner pays the deductible, but Rover can call the vet ahead of time in many cases to take care of payment on the front end. This way, the dog can get their hot spot treated ASAP, minimizing discomfort and the risk of infection.
Allowing the vet to examine it before the owner arrives also minimizes stress on the owner once they return, since they are not rushing their dog to the ER and paying higher exam fees, etc. The vet will also be able to make an educated guess or determination about the cause of the hot spot. If it was caused by something at your home, then issuing a refund is certainly appropriate. However, in some instances, the dog may just be prone to hot spots (my lab is) or have an allergy to a particular tree/grass/insect/etc. In these cases, it would be unlikely that you would need to issue a refund since it was not your fault or the result of your services.