Professionally, because I work in the mental health field, and personally, because I know a few individuals that have a mental illness.
It sounds like the guy in the OP might have something going on mentally but to be honest, more information would be needed to determine if he is mentally ill (specifically what other behaviors does he exhibit that your friend thinks are abnormal. Is he engaging in any behavior(s) that are impairing his functioning. His current psychological state and speech and rationale would also have to be examined)
If your coworker is close to him, or if it's a matter that is very important to her, she might want to see if he can go visit (maybe via family) a professional.