Welcome to JustAnswer, and I'm sorry to hear you are having such a hard time with this. Your mother's symptoms suggest there is alot going one here, and probably more than one system is affected. She likely has renal problems, and may have cardiac problems as well. She could have a chronic renal insufficiency, or even SIADH, which essentially exhibits with symptoms of hyponatremia.
Here's a link to SIADH. This link is from the UK, but I just like the format. Anyway, as you can see when you read this information, there can be many causes for this. Although, in most the patients I have worked with, the syndrome was idiopathic.
At any rate, my main point here is that it is inexcusable for her doctor to "just decide" that she doen't need a referral to a specialist, since he hasn't come up with an answer. You are right to be concerned. Unfortunately in my years of experience with our Elders, this is way too common. Some doctors "just decide" that elders are not worth the effort. In this case it looks like he is guessing about what's wrong, and just doing a trial and error type of approach. Fine for cars, not so good with people, especially elders.
My suggestions to you:
I'm really sorry your mother is having such a hard time with her doctor. This is just pathetic. She really does need a referral to a cardiologist and/or a nephrologist. I wish you well, and good luck to you and your mother. I really wish I could do more for you.