The idea that God is some kind of arbiter of Good and Bad or Right and Wrong is an old argument. Ask your friend if Right and Wrong are a matter of God’s choosing. Can God change what is right and wrong? If God said murder was right, would that make it right to murder? If God says something is right which isn’t right, then God is wrong. God (or anyone else for that matter) can’t make something right just by saying it’s right. Things are either intrinsically right or intrinsically wrong whether God says so or not. All God can (and does) do is reflect what we already know (or knew).
One also wonders, if God is the ultimate judge of Good and Bad, on what set of guidelines, laws or rules is he using to make his judgments? WHY does God judge this to be right and this to be wrong? We don’t let Judges go around arbitrarily deeming people Guilty or Innocent. I have a serious problem with the idea of someone judging me based on a set of guidelines I don’t have access to.
Your friend seems to be using God as a moral kick in this case. He knows the difference between right and wrong, but he also knows he doesn’t always do what’s right, so he’s got God there to give him a moral kick every once in a while.