I have seen a person almost die from hyponatremia. Read this link for a thorough treatise on this condition.

While all "bad situations" are best avoided rather than treated, getting hyponatremia is one thing that you definitely want to avoid instead of treat. The victim gets extremely nauseous and sick. Even the thought of consuming food/liquids can bring on waves of nausea and vomiting.

About the only field-expedient treatment is to slowly dribble in sodium via nibbling on pretzels and sipping electrolyte solutions. In the best of cases it takes many hours to begin to reverse the more severe symptoms. Even when the victim is coming back around, they will have severe weakness and will be in no condition to walk across the room, let alone walk through wilderness terrain.

About the best electrolyte supplement I've found that could reasonably be included in a hot-weather PSK is EnduroLytes by Hammer Nutrition.