Okay, if the docs can't figure it out, I'm going to assume that they've checked her kidney function, hypertension, and heart, and all checked out normal?
IF that's the case, then the problem is she's highly sensitive to refined salt, getting too much in processed foods, not enough real electrolytes (calcium, magnesium, potassium) and not enough Essential Fatty Acids with Omega 3 and 5 chains.
First and foremost she needs to take parsley. Freshly squeezed parsley juice in a carrot/apple juice would be ideal, but she can take parsley as a pill in the herbs section of a health food store and that way not taste it. Start out with one pill a day, in the mornings, and if no change to her urination, increase dosage.
Take a good supplement of minerals, preferably one dissolved in water like CalMagFizz. See if you can find one called "electrolytes" or even "trace minerals."
Switch from regular table salt
to Celtic Sea Salt
. Stop eating so many snack foods or frozen microwave foods or fast foods that are salty.
For the EFA's, take 1 tablespoon Flax Seed Oil daily, and/or evening primrose oil supplements. Eat more fish.
Cayenne Pepper to increase circulation. Get a tincture and add drops to lemonade and/or foods, or just put a couple drops on the tongue if she can handle the heat! ;)
15 minutes a day should lie on floor with feet raised over head, resting on the bed or couch. Slant board exercises would be good too, as would be detox baths.
Hope that helps! :)