I think your friend needs to figure out what make him happy. Working provide the means to have things, and to some extent the title and power while working. I have said it many times, you may have a job to retire from, but you have to have something to retire to. It sounds as your friend has never defined what he wants to retire to. So working keeps him busy and may fulfill a need he has to be contributing something to society. There are lots of ways to contribute to society besides working.
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Semi-Retired 7/1/16: working part-time ( 60%) for now [4/24/17 changed to 80%]
Retired Aug 2, 2017; age 53