Could be there was something else wrong with her. Maybe she just overcame an illness. Maybe her enzymes or lipids or proteins or something were out of whack. She could have had low potassium, high blood pressure, history of migraines, an infection.
There's a thousand reasons that the water could have triggered something rather than caused something. Anything out of place or not normal on her last medical exam will be brought up as a possibility.
That's true, but there's a doctrine in the law called the "eggshell skull doctrine," which basically stands for the proposition that you take your victims as you find them. Maybe it so happens that 99.9% of the general public wouldn't have been injured by this stunt, but she was--and that's the point.
I'm not saying that the radio station will be found liable, but her other medical problems, if any, won't be a factor.