Sure, used to happen often enough. Blood clots or super low oxygen levels. Young people occasionally had heart attacks - probably induced by blood clots.
Sounds like this young man had untreated pneumonia.
Oh I have heard of young athletes etc falling over but it seems like most had an underlying issue. Tragic for sure. So this young man having this I wonder if there was any symptoms of anything or if he knew was off.
Are there any warnings or anything for people that pass away from this stuff?