Never argue with one in public. Someone not knowing you might not be able to tell the difference.
I had a friend once, who I always suspected of being at least borderline, if not full blown psychotic. We became friends when I was right out of college and new to town. He had the craziest parties, never a dull moment around that guy. After a few years it became clear that he really did have problems, a psychosis. I had to cut off all contact. I was beginning to get dragged into some of the delusions, not that I myself was starting to believe them, but I played a role in his. It wasn’t good. There’s no reasoning with an individual living in a cartoon or a nightmare, or maybe both simultaneously, when you’re just a character in it to them.