"If God doesn't exist then there would be no Atheists so the existence of Atheists proves God exists".
By the same 'logic' if football didn't exist there would be no such thing as not playing football. Therefore it follows that everyone would be playing football... if there was no such thing as football!
Yes, folks. People who think that makes any sense can readily fall for religions.
Let's see if we can get away with telling them that if there were no such thing as Atheism, everyone would be Atheist. We may even be able to get them to campaign for more Atheism so there would be fewer Atheists.