My son in law died in the same manner.....everyone thought he was in excellent health...27 yrs old
Tragic case. Yes, dehydration can be deadly.
My doc prescribed electrolytes for me. I only walk fast on treadmill, pedal stationery bike fast as I can. She even told me best electrolytes to buy: Ultima. Comes in flavors, available at AMZ and possibly your local store.
Now I have big bottle of electrolyte water to keep sipping during all workouts. Makes a big difference.
Prayers for this unfortunate woman’s lovely children and the rest of her family.
Some weblinks on the topic of death via exercise.
http://www.sportsci.org/encyc/suddendeath/suddendeath.html