I don’t recommend correcting the word choices of your spouse, but every year as the start of classes approaches, my wife mentions the need to finish her syllabi, and I invariably reply “syllabuses.”
I’m not on the firmest ground, favoring the English plural to the Latin, but I’d be on softer ground if syllabus were a Latin word to begin with.
Syllabus is Latinish. But no one in ancient Rome would have known what you were talking about if you mentioned syllabi or syllabuses. We have one source for the word—Cicero—but he never wrote it.
The word comes to us as a mistake on top of a mistake. Cicero was writing about labels for his parchments, using a Greek word that he rendered as sittyba. At some point, sittyba was miscopied by some ancient scribe as syllabos, and then it was mistranslated by 17th century lexicographer Thomas Blount. Blount’s definition, cited in the Oxford English Dictionary, was “a Table or Index in a Book, to shew places or matter by Letters or Figures.” It evolved into the way we use it today, as a course outline, probably in the 19th century.
The word, then, is only Latin in that at one point someone thought it was Latin. If you’re OK with the fact that it’s not what Marcus Tullius Cicero meant, it’s not what he wrote, and, had it been read correctly in the first place, it would have been recognized as Greek, then you are free to pluralize syllabus as if it’s a Latin word.
The Associated Press Stylebook prefers syllabuses. Half the dictionaries I checked offer syllabuses as the first-listed plural form, and half say it’s syllabi.
A search of the syllabus versus syllabi trend in Google’s book corpus shows that syllabuses was the more prevalent word in books between 1944 and 1992, but syllabi has since regained its status as the more common choice. Syllabi gets twice as many Google hits as syllabuses.
Truth be told, I only rarely include a margin note when an author writes syllabi, and I never bother if I’m editing in a university setting. But don’t tell my wife that.