"Housetrained" dog went pee inside... ?

That's really common for dogs in a new environment. Dogs learn very situationally, rather than generally. So when teaching a dog a behavior, it has to be taught in a lot of different settings before the dog learns to generalize the concept. That's why a dog may know 'sit' in your living room, but doesn't seem to know it as well outside, or even if you are standing a different distance away from her or at a different angle than she is used to. This is the same for housetraining. A dog may be perfectly housetrained at home, or even at all the places he visits frequently, but in a new environment, especially if he's stressed or nervous or excited, it will take a little consistency before he catches on that he's still inside, just a different inside. If you have a different floor type, if other dogs have been there (and especially if other dogs have had accidents inside) and left their smells, if he's feeling a little off, or if he's a bit unsure of himself and his surroundings - these are all very normal reasons a dog will potty inside. Be sure to give him lots of praise for going outside, and clean up the mess well with enzymatic cleaner to discourage him from going again, and interrupt any suspicious behavior with a happy suggestion to go outside. Since he's learned it before, it won't take long for him to get in the groove.

Cleaning up the occasional mess is all part of the gig, and not something to charge the owners for. If you're operating with a business license, you can write off work expenses, including cleaning supplies. If he does it more than a couple times, it might be worth mentioning to the owners in an "FYI, he had a few accidents, it's no big deal, but he may need a little more training in new environments" way, and if it's a huge deal to you, you can decline to watch him in the future. But really, it's just a hazard of the job and can happen with any dog in a new space, so if it happens again, you just clean it up.