Dogs with floppy ears are prone to ear infections. First, you should contact the owner and see if this is a regular occurrence and, if so, what they advise. Sometimes it may only take a cleaning (organic apple cider vinegar is v.g.) or they may have prescription drops for the condition. If it has gotten to the stage where the dog won't even allow you to touch its ear because it is so inflamed, the best course of action is to take the dog to its own vet, which the owner can arrange. The vet would also be able to remove burrs and such, which can become lodged in the ear.
Rover's insurance is meant for more serious situations and, as mentioned, is only a secondary form with a deductible.