I say drink until you make “squish” sounds when you walk.
But seriously, I’d been trying the Atkins diet (high protein, low carb) for the last month and it’s working well for me. Drinking water in quantity is recommended and I routinely go through 1.5-2 litres a day, plus a 2-litre bottle of diet root beer.
Have your buddy make a doctor’s appointment ASAP. If this is a relatively recent development (you’ve said he’s been doing it about a year now…so I’m assuming he didn’t drink that much before), he could be exhibiting the signs of diabetes. If he is indeed diabetic, the longer the disease goes undetected, the more damage it can do.
Diabetes is nothing to fiddle around with. Take him to the doctor TODAY.