If your friend suffers from a mental illness, then the first thing you should try and find out is whether he is receiving regular treatment or not. If he is receiving regular treatment, then you should provide feedback of your observation to him or his family members, who can discuss them with his treating psychiatrist. You can provide support to your friend and avoid any discrimination or labelling towards him. Remember and reiterate in your friend circle that mental illness cab occur in any of us, and discrimination or making fun of mentally ill or mental illness is subhuman and an unwarranted behavior. If your friend is not receiving treatment, then you should be proactive, and discuss with him or his family regarding the same. Early and regular treatment, combined with care and respect are the best treatment for mental illness. Hope this helps you and your friend..