The Greatest Single-Stock Retirement YOLO I've Ever Witnessed

In 2020-2022 I worked at a small wealth management office as a portfolio manager, I basically just did quarterly checkins with our clients and rebalanced their portfolios. My best friend at the company was an advisor who had been in the business a while. We would often grab a drink after work and his father in law would tag along- he was a cool older guy who loved to talk about the market. The two of them were in a constant debate because the FIL had taken his entire 401k upon retirement and dumped it into one stock that at that time he had been holding for about 3 years. The FIL had a career as an accountant for about 40 years and retired with approx 1 Mil in his 401k. He loved to read through the financials of public companies and had found something in Micron(MU) that had convinced him that it was the only company worth investing in. To this date my friend has been begging him every year to sell and diversify, and to this date my investor friend has been dead wrong to do so. His FIL invested at around $35 a share and has only sold small chunks to pay for a house renovation and a couple other expenses. At this point, I don't see any upside in him holding any longer because he is around 75 and will never spend the amount he already has, so what's the point in holding the risk. But he is the most diamond hand MF I've ever met and if i had to guess, those shares won't be sold until the day my friend inherits them. Fortunately for me, those conversations at the bar back then were enough to convince me to buy some MU stock when it was around $70 a share. But damn do I wish I would've bought more and went all in like he did.