How do you comfort a friend who has just found out his girlfriend of 6 years has been sleeping with his best friend?

I really don’t know what to say to him. He just found out yesterday and he’s been a mess. He’s not a very emotional person to begin with, and I think he doesn’t know how to process all of these insane emotions. He said he feels like there’s no reason to get close to people anymore because the 2 people he’s been closest to for the past 5 or 6 years of his life have totally betrayed him. I tell him that that feeling will probably pass, but honestly I don’t know. I have never been cheated on or tried to console someone close to me like this, and especially right after it happened. He keeps asking “Why?” And I don’t know how to answer! I just keep hugging him when he looks like he needs it, but I’d like to say something to give him some hope, any advice?