What do you do if you suspect neglect?

Good for you for being proactive about your guest's health! If your concerns haven't been addressed and you feel it is warranted, your next step would be filing a complaint with your local animal control. Since you've watched the dog multiple times recently, I imagine you even have photos to provide that demonstrate the changes you observed. It may not amount to much, especially if your animal control (like so many animal controls) is underfunded and overworked, but it helps establish a pattern of neglect if there have been previous complaints, or if more complaints are made in the future.

The only thing you could do is mention it to the owner and that is what you did. After that it is up to them to seek care for their dog. Remember your job is to take care of the dog while in your care, after that there is not much you can do. It sounds like you are being really persistent in wanting an answer. Sure you can ask out of concern but if he is not responding then just end it with "I hope Fido is doing better I was just a bit concerned." Next time ask if anything in the household has changed. Example: I have a regular customer who has always behaved well. One day he came and stayed for a week and he was a whole different dog. I couldn't figure it out. When I gave the customer an update I said, has anything changed from the last time you bought him to my house, he is not acting like himself. Have you left him anywhere else besides my house? She replied, the last time I needed a sitter you weren't available and I took him to PetSmart. AHAAAA! there we go. This dog is high maintenance, I could never picture him caged up so he was having some serious anxiety. I realize this in not the same scenario as yours, I'm just trying to say that there is not much you can do once they leave your house.

There was another post on here about a sitter concerned because the owner would lock his dog in a closet (that was his crate) while he was out of the house but that was not considered abuse. Just be careful what you accuse people of. My Chihuahua gained 2 pounds in 2 weeks, Yikes, I cut off treats immediately. Depending on what type of dog that was, it is sometimes easy for them to gain weight but I assure you mine was not because of neglect, maybe too much affection with treats :)