How can I report dog abuse from someone I'm sitting for?

I've been dog sitting for one person for a while now even though they ask me to walk their dog every other weekend. Every time I go to their house, the dog is always in the basement, in her crate and with the lights off even though there's people there.

The dog has scars all over her paws and her snout because I've been told to use a head halter on her. I went to the house about a month ago and when I went in there was diarrhea everywhere including the walls, I went back last weekend and the walls still had spots of poop.

I can also see her ribs. Last time I was there she was so desperate for food, more than a usual dog. Is there something that can be done?