What should I do if I suspect a client dog isn't being properly fed?

Definitely do Not accuse the owner of anything. As you mentioned, they obviously wouldn’t be regularly paying for services if they didn’t care.

Different reasons could explain what’s happening. In addition to it possibly being a medical matter, Drinking excessive water and abnormally high number of stool outputs could be due to nerves, excitement - being in new place, new scents, new people, other pets, as well as possibly resource guarding the water.

Dogs do not poop 5-6 times in 6-8 hours. There is likely something wrong but not necessarily how or what the owner is feeding the dog. It could be the dog has a medical condition, like IBS and, yes, dogs do get that.

But the most important thing is NOT to accuse the owner of anything. Express your concern about the dog's poop habits and how you've never had or seen a dog that needed to do so frequently. Ask if the dog has a medical condition because that kind of information is critical to your care of it. If the owner has no answers, then suggest that the owner bring this situation up to her vet.

Seeing the dog's ribs isn't so bad, but if the spine isn't sufficiently protected, then that's another problem.

When approaching an owner, it is so very important not to accuse them of anything.