Creation ex nihilo is not the belief that something simply comes to be from nothing whatsoever, for God exists prior to (logically or temporally) what comes to be, and is the cause of creation’s coming to be. It is ex nihilo because the universe is not made out of anything pre-existing.
Your friend has the true insight that there has to be something necessary and uncreated to explain the present existence of the universe. For not everything that exists can have a cause; there must be something uncaused. Some atheists claim that the universe itself is necessary and uncreated and explains its own existence. I don’t think this makes sense; the universe is not inherently necessary, even if it has always existed. There has to be something transcending the universe that is self-necessary and timelessly eternal to sustain the existence of the universe
I look at it like this: the universe is not a thing, but a collection of things, which come to be from other pre-existing things, and which pass away into other things. Some atheists deny this and claim instead that all that really exists is truly eternal mass-energy, crystalized (so to speak) into more-or-less eternal atoms and sub-atomic particles. This entails that what is made out of atoms are not really things, but secondary collections of things (atoms). But if they deny that what is made out atoms are really substances in their own right, they deny that they themselves are real – every person is just a collection of atoms, and not really a thing. I believe macroscopic substances are real things, and the atoms that make them up are not really their substance, but only potentially exist independently.
So if you accept that the universe is made of things that come to be and pass away, and that the atomic parts are not the only real substances there are, then the universe is the collection of many contingent and temporally limited things, even if the collection as a whole has always existed (with substances temporally overlapping each other). It does not make sense to me that the collection contingent things is more necessary (self-necessary and uncaused) than anything that is a part of it. So there has to be something that is truly self-necessary, and not merely a long-lasting chain of things. This self-necessary thing is God.
Many people do not see the conflict between supposing the eternal existence of mass-energy/atoms and the reality of macroscopic substances (like humans). I don’t see how they can both be equally real: either atoms are really real, and we are just secondary collections of atoms, or we are really substances, and our atomic parts have no independent existence, but only a potential to exist apart. I cannot fathom a middle ground.
Also, without God, in a cold, mechanical universe, you have no way to explain why there is any meaning or significance to human actions or any transcendent values like love, goodness or beauty. But this is another argument.
