Saw this one on another site...
Herbert Spencer: From The Proper Sphere Of Government:
I asked one of the members of Parliament whether a majority of the House could legitimize murder. He said, No. I asked him if it could sanctify robbery. He thought not. But I could not make him see that, if these things be always wrong, and not to be made right by the decisions of statesmen, then similarly all things must be always right or always wrong, apart from the authority of the law; and if the right and wrong of the law is not in harmony with this intrinsic right and wrong, the law itself is criminal.
A little ammo for your next encounter.
I've asked him if I were to kill him and the majority of people said it was okay would that make it right. His reply is that if the majority of people think it's okay then it's right. I think the statement you quoted supposes that the person answering "No" must have a Jewish or Christian moral foundation of Thou Shalt Not Murder. To him there is no "intrinsic right and wrong".
I've come to believe that a person who does not believe in God can't be made to believe there are absolute right and wrongs. If not for God's commandment, what makes murder wrong?