If he's like me, that's not the case. I've always drank tons of water after drinking booze. I used to not get hangovers, but as I got older the hangovers got worse. I took a few years break from drinking, and now when I try drinking a little the hangovers are worse than ever before, no matter how much I hydrate. Two drinks at dinner is my limit now. And no earlier than dinner, because it makes me very lethargic for the rest of the day. Three drinks and I'll be miserable for the next day too.
You have alcohol tolerance and I no longer have that. Your experience with hangovers is entirely based on tolerance.
Hydration and food are an important aspect of not getting a terrible hangover for me still but it does not stop them entirely.