First, I'd point out that according to the three leading indices of freedom, only 13 nations (out of almost 200) are more free than America. For all its flaws, America's constitutional republican framework has been by far the most successful in human history. It has been increasing personal and civil liberties almost monotonically for two centuries. In just the last half-century we've seen enormous progress regarding racism, civil rights, divorce rights, sexual freedom, reproductive freedom, gay rights, criminal procedure, free expression, gambling, and even society's attitude towards substance use. Meanwhile, America is among the most economically free nations in the world, with a per-capita GDP exceeded only by Norway and Luxembourg.
Second, I'd quote his own words back to him: "To secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness. Prudence, indeed, will dictate that Governments long established should not be changed for light and transient causes".
Third, I'd inform him that if he thinks anarcholibertarianism depends for its justification on the notion of a personal Creator whose whims dictate our morality, then I'd tell him to read the academic literature regarding the http://en.wikipedia.org/wiki/Euthyphro_dilemma.
Finally, I'd warn him that there is a place called the Internet with people roaming it who say that debating them is comparable to debating a time-traveling anarchistic Thomas Jefferson. :-)
If you're not sure how or whether I count as an atheist polemicist, see http://www.infidels.org/library/modern/brian_holtz/bio.html.