This may not help much in this case, but I do know that sodium competes with potassium in the kidney tubules, so if she likes bananas and other potassium rich foods, or she is getting medication with potassium in it, I wonder if perhaps this could be a reason? Its a bit of a long shot though.
It sounds like she's not getting enough sodium in her diet.
If she's drinking lots of fluids then she might be flushing out too much sodium & more.
I've had trouble keeping my sodium level too the last several years, resulting in ileus & doing things I had no memory of the next day. Adding extra salt to food wasn't enough & after my third hospital stay I began taking at least two grams of salt pills daily + salt on food. That's seems to be working well enough for over three years now. The bad part now is that my bp is high.
If she's taking HYDROCHLOROTHIAZIDE for high blood pressure that could reduce her sodium level & explain her ******* ( http://blood-pressure.emedtv.com/hydrochlorothiazide/side-effects-of-hydrochlorothiazide.html )
yes too much potassium gets rid of sodium in the body. I think the doc needs to look at that. She may need a pacemaker if the bp is too low.