
If they are concerned then it would help if they can put into volume how much the dog drinks per day, by measuring.
If it is a behavioural thing then they need to somehow record how many times the dog goes to the bowl, how long he drinks for, and also how many times a day he pees. Plus the volumes of water, that would help more.
Hard to know if this is a medical thing or a habitual thing, but if they were happy to do a bit of recording it would probably help.
For the record my rescue girl drinks loads - she is always drinking it seems (and dribbling water over the floor) plus when on walks she pees every 30 seconds.... yet she can also go 12 hours or more without a pee... and sometimes it's a full scale performance to get her to wee when you know she must be bursting! She is raw fed too, so she gets lots of moisture from her food yet still drinks a great deal. So I would say in her case, it's psychological - perhaps in her past she was deprived of water.
My other dog whom I have had since a puppy and is now raw fed, seems to rarely drink water except when he is out & about and running & getting warm. He is a good example of a dog who has never known water deprivation, and indeed he is great at smelling out water sources by himself when we're out walking, if he is thirsty. He also knows the word 'drink' and will either look for water, drink on command, or come to me for water if needed.