Nobody likes the devil's advocate, but I'd probably spread the word if I thought someone was dangerous. The biggest thing is to make sure that you really have a reason to. Part of the fun of being online is that you can fudge details a little if you wish, it's not that out of the ordinary for things be different in the flesh.

So if the lie was a whopper and his intentions seem to be outside the range of acceptable oddity, then speak up. But definitely don't get your heart set on being thanked for your honesty. A lot of people will probably question your intentions and motives for talking about him.