[cferr]

Caffeine is a diuretic. If he isn't drinking decaf, it's a net loss.

[SketchySeaBeast]

While it is, it doesn't lead to a net loss in hydration.

https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-h...

https://nationalpost.com/health/nutrition-fact-or-fallacy-do...

http://time.com/5192272/coffee-tea-dehydrating/

https://www.webmd.com/diet/qa/does-caffeine-have-a-dehydrati...