How do we know the F.O. was the sane and balanced one?
Because he's not the one who ran through the cabin telling passengers to pray to f***ing Jesus Christ and who rambled on about 9/11, Afghanistan, Iraq, the sins of Las Vegas, push the throttles to the stops, etc.
Also because the F.O. was the person who landed the aircraft and who had the wherewithal to ascertain the situation and remedy it.