Human beings are like clay, and the life we live molds us.
After all, the very language and accent in which we speak is just our brains being molded by hearing others speak.
Your friend was molded by his life into something different from what you knew, and I don't know if that's good or bad. It could be either.
In fact, that TV show, "Breaking Bad" is all about this, how circumstances can mold a mild mannered middle class chemistry teacher into a fearsome and terrible drugs kingpin.
I have come across some old friends recently, and some seem haggard and gaunt, and others have spring in their step. What they did in life changed them.
However, whatever these changes are, they are better than the alternative. One of my closest early friends suddenly died of an aneurysm. He was a food critic for a newspaper, always so full of life. I wish he had had the chance to change.