naturally i can't speak for the person you know, but i think violent video games can be used as a helpful outlet and a way of diffusing irl anger and frustration. when angry at issues in real life and frustrated over the way things are going, turning on a video game can prove to be a helpful distraction. however, it's probably not safe to channel all of that into a very violent game, and in certain individuals it could easily create more aggressive behavior. i've certainly seen people get very aggressive and angry if someone distracts them while they're focused on a game, and get angry when they're away from the game and unable to play. there's a line that's easily lost and people can get too lost in the virtual word, in my humble opinion.

if a lot has gone wrong in his life, is he seeing a therapist? therapy could be helpful in this situation. i tend to think sometimes people unjustly blame video games when that's not where the blame is due. i can say i enjoy war games on the computer and while there's some i can't play because they're too violent or too scary/disturbing, there are others that are a helpful distraction and destresser.

i did a quick google search and didn't find anything that looked helpful, i'm sure someone else will be able to provide some decent advice and perhaps some useful resources.