Thank you for the correction. He may prove that the human race is ultimately doomed through stupidity but why upset someone if they are happy? Is it really worth it in the end?
I guess it comes down to personal preference. I would rather hear I'm very wrong from someone who I know than a group of strangers who will, more likely than not, be very cruel.
I agree with 4, to a certain extent. I would definitely tell someone they were wrong and explain it to them, but if the person kept disagreeing and didn't believe me even though I've tried to show them proof (through Google or another person or whatever) then I'd just drop it. If the person really wants to believe they're right in that situation then who cares. It's not like it's a topic about something important in life and it doesn't effect anything. There's no need to continue with a huge argument if the person is being stubborn. Let him think what he wants at that point.