A neighbour of mine died last week a moment after reaching his climax during the sexual intercourse with his young partner. He was 79 years old.
Is there any available English word to describe his sudden death?
With the facts of your case, I would say, another expression would be:
I was shocked by my neighbor's unexpected death.
Your new question:
Can I use the word "breakstroke" for this case ?
The normal term is just "stroke," the result of a rupture of a blood vessel.
Perhaps someone else has heard of one, but I think that the shortest phrase you can get to describe a sudden death is, in fact, sudden death.
A relative of my friend's died suddenly after one night; he was badly drunk that night. I don't know why he died, I guess it was related to the malfunctions of blood circulation and brain's activities. Sadly, it was an unexpected sudden death.