I've heard about this condition before. It can occur in exercise or in hot weather, when sweating causes you to loose salts and minerals. If you drink lots of plain water while exercising (which does not contain salts or minerals, as opposed to stuff like Gatorade), you're diluting the salt that remains in your system. Screw up the salt balance in any living thing is an easy way to kill it. That's why putting fresh-water fish in ocean water or salt-water fish in fresh water kills both. Their cell walls explode.
The woman's death was most unfortunate. Lots of radio stations (and radio listeners) do dumb stuff like this without even bothering to think about what the consequences could be.