How do I handle when an owner misrepresented their dog?

Yikes. Day 1 is always the hardest for dogs to settle in with a good number of dogs having accidents despite frequent walks. Given all the information here, I'd suggest contacting the Rover Support, especially if this leads to a discussion of other options for care. I'd also suggest contacting the dog's owner. At 8 months, I'd guess this may be his first time apart, so you could delicately approach the behaviors by stating you think he may be having separation anxiety and you want him to be happy, so this is what you're doing (3 walks a day, etc.), but this is what is happening in the home... Do they have any suggestions? If you've already spoken with Rover Support, you could also bring up possibilities that they may want to consider.

First thing I would do is contact the owner and ask if any of these behaviors are normal - sometimes, dogs, especially at such a young age, act differently than they would at home, as a sign of separation anxiety.

If it continues, contact Rover Support, they may be able to assist you further.

Ditto what Deb and Daniela said. It's not at all unusual for a dog, especially a young dog, to show vastly different behavior at home than they do at a sitter's. A dog that young may not have had many chances to generalize their house training to include locations other than their own home and maybe a select few other areas they spend a lot of time. Like with any behavior, house training has to occur in a variety of settings under a variety of conditions before a dog learns that indoors is always indoors, even somewhere they've never been before. Similarly, a dog who has learned to sit in their quiet living room probably won't sit on cue in a busy park.

With any new guests, the very first order of business after they're dropped off is taking them for a walk and giving them lots of genuine praise for pottying outdoors. Even with an impeccably trained dog who hasn't had an accident in years. I'll continue with the reinforcement throughout their stay (I do it with my own dogs, too), because it takes me no effort at all to happily say "Good girl! Thank you! Good 'go pee!'" and it helps prevent a lot of accidents just by showing the dog what I want them to do before they have a chance to do something I don't want.

I'm glad to hear the owners are taking responsibility; it will be good information for them to know in the future to help prepare any future sitters.