There are plenty of diseases where you die within a few months of diagnoses. For all we know he got his diagnosis and immediately took his leave.
While I agree with your overall sentiment that it's not worth it, you are making so many assumptions as you know nothing about his personal life. Not everyone has a good family life, and it's a dangerous assumption to blame work for it - the causality can often be the other way round (crappy family relations, spouse unwilling to change, etc so find fulfillment in things outside of home).
We just don't know. And it's none of our business.
I don't believe there's enough information to reach that conclusion. It is quite possible that his sickness was sudden or that it was discovered suddenly at which point he took a leave of absence. There are numerous people who go into the hospital for a routine check of - I feel tired/I have a constant headache, etc only to be told they have a life threatening disease. By the time the father of a friend was diagnosed with lung cancer, it was already stage 4 or so and he stopped working then