So as someone who likes to overindulge per occasion and has had many, many bad/severe hangovers I have found the things that effects me the most. Like my hangovers were so bad that I normally couldn't eat any food without throwing it up again, so I ended off sleeping the whole day and only felt better the late evening. Depending on how responsible your friend is, water after every drink is a nice thought, but really is unrealistic for most people drinking to have a good time.
However most people get really hungry after a night of heavy drinking, and that is the saving grace between feeling like death the next day and only feeling thirsty and tired the whole day. It sort of levels your stomach so to speak so that you don't feel like total garbage the next day, some people can speak more to the Ph levels of your stomach and how drinking effects it. So my advice. Drink as much fluid and eat a nice meal before passing out. When you wake up repeat the process, eat (anything heavy) and drink(a lot of fluids). Depending on how much fluid you drank before you passed out, it might safe you from the next morning headache (due to dehydration). If he needs to urgently function the next day, a slowmag or something should carry him through the day fine.
Another pro tip: Before you pass out, make sure you have a large glass of water next to your bedside.
If all else failed, take the day and just sleep it off.