There is no flaw in his argument. He's given to a very aggressive skepticism.

The question I would pose is what grounds would you havefor adhering to the conjecture posed. If you accept his argument, what about your behavior should change? Can we derive any knowledge from his conjecture? If "no" and "nothing", then it's a valid question why we should believe it. If he persists, point to David Lewis and linguistic assent (outlined, wuite entertainingly, in "Elusive Knowledge"): that he can consider your view and his completely equivalent insofar as either of you have knkwkedge of any kind,.

Also, excuse my drunkenness.