@Borderline
The human stomach typically holds about half a gallon. Stretching it out to a full gallon is painful and the body will try very quickly to unstretch it as soon as possible, usually by vomiting. This is where the "1 gallon challenge" plays on. You actually DON'T wanna keep that milk in your stomach for the whole hour, it's dangerous.
In this case, a gallon of water, especially one that's thrown up, is fine. Water toxicity, historically, has kicked in at around the 2 gallon mark. Most stories about it happening (people drinking water heavily in gyms, that one "hold your wii" tragedy) involve people drinking around 2 gallons. That said, Emily's a child, so for all I know the LD50 of water could be a single gallon for her. But again, this is assuming it
stays in her; vomiting diminishes the effect.
Another thing to keep in mind is that this is the danger of drinking that much
pure water, because it's basically stripping out electrolytes as it runs through your system. If it was a gallon or two of say, pedialyte, saline solution, or gatorade, she'd be fine. Those have about 2 to 4 grams of sodium per gallon. (Oceanic salt water is about 10x that, and brings us back to death town)