Allow tracking of friends' data/graph. Allow remote 'nudge' if they aren't consuming enough.
Why is there even friends if you can't help them stay honest. Notifications are great but you should have the option to nudge each other/remind them to drink some more water. That's what friends are for!
Like this idea ... I joke with my 16-year old daughter that I'm her "hydration coach" as she tends to go all day without drinking water then expects to play two 40-minute halves in a soccer game and wonders why she runs out of steam!
Love this idea! My wife and I are trying to encourage each other to drink more water so it's a nice friendly competition idea throughout the day when we're at work!