Hi Darius, I'd like to help you with your question.
There could be a couple of things going on with your friend. One, he could have late on set schizophrenia. While it is rare a person develops schizophrenia after the teens or 20's, it does happen. It is thought that developing schizophrenia at such a late time may indicate another underlying problem, such as early signs of dementia or a life long problem with mild schizoid symptoms that have not surfaced before this time.
Your friend could also have a physical disorder. As I mentioned, symptoms of dementia can be confused with a mental health disorder. Also, there is a disorder called Pick's disease that affects the frontal lobe, which affects reasoning.
The best option for Mark is to have him see his doctor. It should be easier to suggest to him that he see his doctor because he does not seem well than to suggest seeing a psychiatrist. If Mark is willing to see the doctor, you can call ahead and explain what you have been witnessing with Mark's behavior. You can also offer to go along, if you feel comfortable doing so. The doctor may not be able to share his/her findings with you (because of confidentiality) but at least the doctor can refer Mark if his problem is not physical but mental. You can also ask if Mark is willing to sign a consent to allow you to become involved with his exam and treatment, if you feel you want to do this.
I hope this has helped you,Kate