I have a friend whose very much mentally ill is there anything I could say to him or do to help?

I have similar friends with these difficulties. I think the best thing you can do to help is realise that it is no good arguing with him about his beliefs. if you do this may cause him stress, which he cannot afford to have. Tell him that you respect his beliefs but you may differ with in your own opinions, but do not wish to argue. if he is stressed try to calm him down, and suggest an easier subject. Perhaps you could play cards or work out a quiz. Good luck with your friendship.