But in one case she reveals her youth. Typing two spaces after an end-of-sentence period, she thinks, was only done on typewriters because typewriters have monospaced type, and you shouldn't do it on a PC.
Like many theories, it sounds logical, but those of us who read old books or are old enough to remember when typesetting was an art practiced by people, rather than the result of an algorithm, know better. Typists were taught to hit two spaces after a period when typing because typeset material once upon a time used extra space there. That was not all that long ago either. Looking at random through my library, I find a work copyrighted as late as 1962 (University of Notre Dame Press) that still has extra space after the end-of-sentence periods; and the further back you go, the more works show it.
In fact, it is almost possible to date the changeover with some exactness. The January 1967 edition of the U.S. Government Printing Office Style Manual contains rule "2.40 To conform with trade practice, a single justification space (close spacing) will be used between sentences." A footnote, however, adds "This change was approved after the Style Manual was revised and, therefore, is not followed in this printing." As indeed it is not: a quite considerable space comes after each end-of-sentence period. Similarly, the second edition of Words into Type had, I clearly recall, a discussion of the question, in which it was noted that educators in particular favored the extra spacing, as an aid to making the sentence an easily visualized entity. In the third edition (1974), there is still an index entry for "spacing?-punctuation" and the page cited interestingly contains the statement "After a question mark or an exclamation point within a sentence the spacing should be less than that used after periods, usually an en if sentences are em spaced." But nowhere is there a directive to add the extra space after the end-of-sentence period.
