To be totally honest, the owners with their mice (arguable, but still hinting at weirdness so it adds up) and sprinkling dirt on the dog's food, keeping the dog too thin, and taking water away from 4pm to 6am sounds creepy and wrong as heck. I'd bring it up with the owners too, but they sound like they lack reasoning or logic as normal people to begin with.
Some people are just crazy and wrong and there's not much for a doggy daycare person to do about it, if the owners choose not to listen. Many people who shouldn't own animals, have animals. But if the animal isn't in extremely poor condition to qualify as abuse, other people can't do much about it except feel sorry for the animal for having neglectful owners. I'm sorry you have to witness this. :\