Claiming German citizenship through Grandparents

I believe it's not possible, because based on German Law(if I am not mistaken) the citizenship of parents in the time a child was born is the deciding factor. I do know there are cases where the grandparents were German and the parents also (automatically) German but never registered as German bec. born and living abroad. In that case I believe its very possible for your friend to get this citizenship, even in the case of grand-grand parents, as long as no one in the line ever renounced it(not using it and not registering it doesnt meant renouncing it) BEFORE your friend was born. I believe your friend has a very weak case, however, perhaps a good lawyer may help(to dig into why his father was renouncing it, etc, maybe there is something that can be used to make a case. This is what I know. Hope that helps.