In Which Stephen Fry Steps In It

Wilson makes a good point: In Which Stephen Fry Steps In It |:

But I want to note something really strange about this set up. When God and Fry take their places as these disputants, Fry undertakes to argue morality with Him. And in order to argue this way, he has to assume — and most certainly does assume — that there is a moral standard that overarches the two of them, and which is equally binding on both of them.

 But what is that standard? Where did it come from? How does it come to be binding on both of them? In order for the standard to be authoritative, there must be an authority, correct? Who is that authority?