My friend has been diagnosised with an unspecified mental disorder. He was originally diagnosised with schizophrenia as a teen. Why the difference now, later on in life? He is 50 years old.

Maybe he doesn't exhibit those symptoms any longer. That can happen -- we can "grow out of" a diagnosis--simply maturing plus medication and therapy. My autistic son at 6 became a lot different after lots of one-to-one help. Now he is 42 and hardly seems autistic at all.