I have a gsp, which are thin dogs to begin with. She was incrdably skinny until she reached two years. She had ribs and spine showing at all times, but she was fed as much as she would eat, which was about three cups a day. We also gave her a big can of wet food to try to get her to gain weight. Bet wasn't concerned as he knew we cared for our dogs and that she was given a lot of food if she wanted it. She is a very active dog, as she is a hunting guide
dog. I would say if the dog is super think talk about your concerns with the owner. Larger double meals will cause the dog to retain more weight than a bunch of small meals. My guess is you are correct and the owner is clueless, but you never really know. I know a guy who runs sled dogs and they are all really thin but eat whatever they can take in. Maybe there is more to the story than we know.