The Big Bang theory was invented by George Lemaitres, a Catholic priest and physicist, not a group of basement atheists. In fact, physicists opposed the big bang for some time precisely because the standard model posits that the universe began to exist (which is basically the modern consensus).
The big bang says nothing about the cause of the universe, just that the universe seemingly began out of nothing and started expanding from there. The term 'big bang' really only refers to the history of the universe after the first instant of the beginning of the universe, and this beginning could be caused by anything, including God. The big bang does not a priori disinclude God. In fact, it has nothing to do with the question of God -- it leaves the question open. Your friend seems to have conflated the big bang with naturalism.