>>He is 87 and doesn't think recoding a super niche' software tool is the best use of what very well may be his last few years of life.

Know quite a few elderly men, were moving mountains until retirement, then at one age they wanted to simply step back and relax. It was a cognitive downhill from there on. Also there is something strange about men sitting at home doing nothing. For some reasons families start hating as little as a sentence from them. You have to sit quiet for most of your life. Which honestly speaking is nothing short of a punishment, because you are actually expected to behave like furniture, or at best like a vegetable.

Even other wise I do see men who retired early not having all that a great time sitting at home and doing nothing.

Without a purpose, you won't enjoy living life much.