It's different for everyone, what they did, what they saw, what evils they take back with them, at a deep personal level.
What worked for me, may not work for others. A big factor for me was something simple. The question of WHY. Why was I mad, anger, upset, whatever. I would answer with excuses at first, to justify it to myself.. after awhile, I couldn't answer my own question.
Once I was able to do this, I was able to be helped by others. I would allow the help, listen to others, and so on. The only thing I can say that brought me to where I am today is my business partner and my wife. Without them, I would still be lost.
With your friend, you can ask him why
and you can wait.. and once he finally breaks, then you step in as a friend, get his family, and everyone rebuilds him - reteaches him, love.