If you're looking for facts and a well presented intellectual argument, it's hard to go wrong with works by John Lott. I recommend:
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More Guns Less Crime, University of Chicago Press, 1998, 2000, 2010
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The Bias Against Guns, Regnery Publishing Inc., Washington, D.C. 2003
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Straight Shooting, Firearms, Economics and Public Policy, Merril Press, Bellevue, WA 2006
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Here's one by Richard Poe: The Seven Myths of Gun Control, Three Rivers Press, NY., NY 2001
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If she's intellectually honest she'll find it difficult to overcome the facts presented in these books. As for "why a gun, all the time?," in addition to some of the recommendations made in other posts I might move her towards some of Colonel John Dean "Jeff" Cooper's writings.