What she wanted to know was how inexpensively her book could be professionally edited in that she and her friends and colleagues have probably now identified most, if not all, of the remaining errors and this would be a quick job as kind of insurance.
Her question was good but her understanding of the editorial process was flawed. Think about it this way: I already have a bag of flour in my pantry so I probably don’t need another bag, but maybe I’ll buy another bag just to be sure. Surely this just-in-case bag of flour should cost significantly less because I don’t really need it; the bag I already have is enough. Try that line of reasoning on your grocer and tell me how you fare.