What to do with dog who pees ONLY IN THE HOUSE?

Hello! I have a dog that really only pees IN the house. His humans say he only does it when he's boarded or away from them, separation anxiety they say. He drinks an unnatural and copious amounts of water than any other dog I board and literally just pees as he walks. Even pees in his sleep. I'm told he has to wear diapers when he's with me. Yikes! That's a lot of pee-filled diapers.
Last stay they left me one diaper for 5 days.! I've discussed this with them more than once and they say casually,- "he doesn't do it at home!" Like they don't believe me. Here's my dilemma: I do not know how to tell them I don't want to take him anymore because of this as I really like all of them. Appreciate your opinions and Help!!