You are a good friend in helping to learn more about it in support.

There typically can be some kind of underlying disease or hormonal process that can cause the unbalance or an issue of fluid balance causing the problem. That should be evaluated to find while treating the symptoms. There are so many things that can cause hyponatremia. It should not however go untreated. You could encourage her to find a doc who will help if her current doc will not.

Perhaps this might help explain better in this link to the Mayo Clinic; it discusses the definition, symptoms, causes, risks, testing, treatment, etc.:
Hyponatremia Definition - Diseases and Conditions - Mayo Clinic