Suppose his existence actually has no 'purpose'. Surely he is then correct, and you misguided? After all, you can't give him any evidence of such a purpose; it would seem to me that he is merely being sensibly skeptical.
Our we applying this condition to an individual atheist or atheists in general?
If so, is the point that there is no purpose to atheists?
Or is the point that there is no purpose to people in general?
What we end up with is a faith that there is no God and no purpose to their existence.
Fortunately, you are wrong. Unfortunately many people hold this faith.