Earlier this year, Leslie Hazleton, an English journalist, wrote a biography of Muhammad called, First Muslim. Some critics questioned her ability to write an objective biography, but none seemed as obtuse as the Fox News anchor who grilled Aslan. Dan Murphy, writing for the Christan Science Monitor, takes up this question in this excellent article and Susan Brooks Thistlethwait, writing for the Washington Post, deals with the question in an equally brilliant manner. I highly recommend both articles.
If we follow Lauren Green's line of reasoning, only adherents of a religion are qualified to write about them. A westerner cannot write about Hinduism or Buddhism with objectivity, regardless of the degree of their scholarship.