Yes very sad. Same thing happened on a CO flight from Mexico to IAH some years back. These days it is possible to survive a massive heart attack - but you need to be within 10 minutes of a major medical facility and in the hospital within 20 minutes. Obviously that can't happen when you are 40,000 feet in the air.
This is why a crew member always goes in the cockpit when one of the flight crew members leaves to use the restroom. Can't leave just one person in the cockpit inside the locked door.