I responded with a simple "Nope. Never noticed." If he decided to create an argument about it I'd take him up on his offer.
I've never met someone "immoral" because they were atheists; in fact usually the most immoral people I've had to deal with claimed to believe in god. I was thinking of explaining how mandatory religion usually results in terrible things... But your simple response seems best.
I'd never use this argument seriously because correlation does not equal causation, it has selective observation written all over it, and it's a matter of opinion, but never the less I might respond with something like this just to make a point on how ridiculous the argument is:
"Ever notice how when God wasn't optional people believed the witch trial burnings, crusades, stoning, oppression of women and minorities, slavery, and things like that where morally right? Ever notice how when God wasn't optional society was savage, ignorant, infested with disease, and things like the dark ages happened? Now, ever notice how slavery and murdering people because they believed something different then you became immoral and equal rights came alone right about when God became optional? Ever notice how society produced things like flight, space travel, improved medicine, cars, the internet, indeed almost everything you use, oh and not to mention doubled your life expectancy, right about the time when God became optional?"
haha, nice one
Need someone to scare the infidels off your lawn again???
That which doesn't exist, is not an option.