What happens if you drink a gallon every day?

That's 16 cups of water. She'll probably be okay if she spreads it out throughout the day. When I was a teen I read that water would clear up my skin and drank 8 glasses at one time and vomited. That's probably what will happen to your friend if she drinks too much at one time. That's a lot of running to the bathroom--I think 8-10 cups would be enough if she's trying to keep her system flushed out and keep herself hydrated.