More on the tragedy here. Some quick calculations. Assuming she weighed 63.5 kg (140 lbs), with a 50% body water percentage, her total body water was 31.75 L.
According to this article, she drank 2 gallons (7.5 liters) over approximately two hours, and we'll assume her kidneys eliminated 1000 ml of that -- figuring her max urine output was 500 ml per hour, and a full bladder holds at most about 1000 ml -- which means her total body water increased by about 6.5 liters in two hours (assuming that she absorbed all the water she ingested). If her serum sodium was initially normal at 140 mEq/L, after the contest she would have a serum sodium of [140 X (31.75 / 38.25)] = 116 mEq/L.
Serious symptoms usually occurs when the serum sodium is less than 120 mEq/L. Dropping your sodium from 140 to 116 could definitely lead to brain swelling and death.