I take your point, Jan, but as I understand it a heart defect is a specific disorder.
I wouldn't file that under 'natural causes'.
As I understand it, natural causes is used when someone is 70 or 90 or 100 and their heart just stops beating and they die peacefully in their sleep.
I don't think this happened here.
(BTW, I'd like to echo Unky Punky's sentiments - I hope this man's loved ones get comfort and support at this difficult time. I should have said this in my original post.)