Welcome to NeuroTalk.

The elderly do get low sodium spells. This is called hyponatremia. But drinking MORE water is usually not the solution to that. So I am confused about that advice.
If she were just dehydrated, that would be a different situation.

Here is more on the subject
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1071707/
Antidepressants in the SSRI category also may cause this but rarely.

Also, some elderly patients when traumatized develop low thyroid in response. So I'd make sure they did a thyroid test on her while she was in the hosptial. Low thyroid can give mental confusion symptoms.

You know when a person hits their head, the brain can move about inside the skull and other parts may be damaged.
There is also a condition called normal pressure hydrocephalus:
http://www.lifenph.com/
This can present with cognitive issues, incontinence etc. The blow to the head may disrupt the circulation of spinal fluid, and if it builds up it can cause this.
http://www.lifenph.com/symptoms.asp

It can be very difficult with a very elderly patient therefore.

I think you'll need to see more doctors, and perhaps get another opinion.