Whenever I introduce guest dogs to my home, I do an extensive meet and greet process to get them introduced to the "pack", which is vital to preventing any aggression throughout a stay when you have resident dogs. Your home is personal territory to your resident dog, and to any dogs staying there. When a dog walks into the home without having become acquainted to the other dogs, sometimes things are tense! My best recommendation would be to get them out of the house to meet each other on neutral territory. That way, when they come back in the home they can feel more like a pack. I'm not sure how many dogs you have in your home right now, but it doesn't have to be all the dogs at once. I would start with two, maybe the two that seem to have the most tension between them - take them on a walk - a long walk, if they can manage a longer walk - walk them close to each other if they will tolerate it. You want them to feel like a pack, like a dog sled team - like they are working together to accomplish something. By the time they get home, they will hopefully have been so focused on the "task at hand" that they will have forgotten about the tension between them. Then they can walk through the door together and that act of entering the home together is SO BIG!
Do as much of this as you can. Walk the "problem" dog with all other dogs in the house, on one on. Walk all the dogs together. Get as MUCH tension out of that house as you can. Get as much energy out of the dogs, at the same time!
If this does not work, you have options! Contact the owners! There is no shame in saying that it is not working out. Maybe they have a friend or family member who can take the dog. Rover support can also help out by finding a replacement sitter. Do not be afraid to contact Rover support, this is why they are there! You shouldn't have to deal with a dog that is just not working out! Sometimes dogs aren't a good fit, that's just the way it is -- you can do everything in your power to make it work, but there is a point where you have to address the facts.
I wish you the best of luck! Definitely stay on the safe side and use your gut instinct. If things are not safe with the dog in the home, contact the owners and Rover support.