His entire life and a very large part of identity is in the process of being attacked and destroyed. You will see a lot of changes in your friend, most likely.
Offer support but keep your distance. This is much bigger than you think, and the divorce process will probably last several years. If he survives that, he will have to figure out how to pick up the pieces of his life and start all over again.
When mine hit my entire adult life was rebooted. I was back to living in someone's basement, living off minimum wage. I lost every penny, every stick of furniture, the big house, the car, most of my time with my son and a large portion of my sanity. The only thing I had left was my job, my family and looking forward to time with my son. I was also lucky enough to meet someone new who was also going through the same thing and we supported each other... without getting too involved in each other's divorces.