I mean, you are making a jump from correlation to causation. There is a reason science does not use anecdotes and relies on statistics, p values, and, confidence intervals.

That being said, did you happen to know what ingredients were in play, since he was using the same product for ~>20 years it sounds like, maybe the carcinogen landscape was much more under researched that long ago? I mean leaded gas was still sold in the US 'til 96 -- even though theres just so much evidence of it being harmful to the nth degree.