If he's had a moan to someone who has agreed with him, well people are prone to take that like some concrete confirmation even though they know the other person at best only has the same limited info that they do.
Once people think their views have been verified they'll pretty much disregard all information that runs contrary to it, and anything that can possibly be twisted into supporting their idea will be so twisted. If someone is challenging their position then the effect is even stronger, to the point that people feel justified in making shit up because they're so convinced they're in the right even once they've admitted to themselves they don't have any facts to back up their argument at all.
IIRC it's fairly basic psychology though dunno what it's called to google an article. What was I saying about being sure of things even after realising you don't have facts? Connected to confirmation bias and rationalisation though.