Five possible places to start
ONE: (sarcastic) What, are you telling me that we have a Constitutional Amendment especially to say that the government may have weapons? Wow.
TWO: ""A well-crafted pepperoni pizza, being necessary to the preservation of a diverse menu, the right of the people to keep and cook tomatoes, shall not be infringed." I would ask you to try to argue that this statement says that only pepperoni pizzas can keep and cook tomatoes, and only well-crafted ones at that. This is basically what the so-called states rights people argue with respect to the well-regulated militia, vs. the right to keep and bear arms." – (quote from Bruce Tiemann)
THREE: "the people" means the people in each of the other amendments. Why do you think it doesn't mean the people in this one? (probable answer, "the militia..." and you can then segue into the pizza thing above or the book one below).
FOUR: A well-schooled electorate, being necessary to the security of a free State, the right of the people to keep and read Books, shall not be infringed. Would anyone argue that this means that only people who are actively enrolled public school students should be allowed to own books?
FIVE: Show them Amendment Three, which reads, "No Soldier shall, in time of peace be quartered in any house, without the consent of the Owner, nor in time of war, but in a manner to be prescribed by law." If the Founders had intended to allow any infringements (infringement means nibbling away at the edges of the broad right) -- they had the language for that already to hand. It's right there in the Third Amendment: "but in a manner to be prescribed by law." While this doesn't directly address the militia/non-militia question, it does go right to the heart of the entire gun-control mindset.
pax
False is the idea of utility that sacrifices a thousand real advantages for one imaginary or trifling inconvenience; what would take fire from men because it burns, and water because one may drown in it; that has no remedy for evils, except destruction. The laws that forbid the carrying of arms are laws of such a nature. They disarm those only who are neither inclined nor determined to commit crimes. Can it be supposed that those who have the courage to violate the most sacred laws of humanity, the most important of the code, will respect the less important and arbitrary ones, which can be violated with impunity, and which, if strictly obeyed, would put an end to personal liberty - so dear to men, so dear to the enlightened legislator - and subject innocent persons to all the vexations that the guilty alone ought to suffer? Such laws make things worse for the assaulted and better for the assailants; they serve rather to encourage than than prevent homicides, for an unarmed man may be attacked with greater confidence than an armed man.. They ought to be designated as laws not preventive but fearful of crimes, produced by the impression of a few isolated facts, and not by thoughtful consideration of the inconveniences and advantages of a universal decree. -- Cesare Beccaria. Thomas Jefferson copied this passage in full in his Commonplace Book