Friends with PTSD

I just came back from Wyoming. While there I visited a friend/neighbor of my mother. He served with the National Guard in Iraq 2 years ago. He made the comment that he should have taken a gun to his head and blown it off while he was over there and maybe he still would. It frightens me that he has those kind of thoughts. When he talked about it, his demeanor turned real dark. I myself have had thoughts of suicide. But I have never gone to the extent of actually getting the tools/devices to do it. What is it in the human mind that makes us feel this way? Is it a chemical imbalance? Well I worry about my friend and I want to be there for him. But he is the type of person that doesn't want any kind of help or concern for his well being. Kind of like "I'm a man, I'll deal with it in my own way in my own time." But I really care for this man. He shows great concern for my widowed mother. In fact he does more things for my mother than my "lame" brother-in-law who lives a block away. I only hope and pray that I can be of some great help to this man and help him somehow through this PTSD.