Q: If someone referred to as “She Who Must Be Obeyed” becomes the object of a preposition, should it be “She” or “Her”?
A: We’d treat the noun phrase “She Who Must Be Obeyed” as any other noun. We’d use it as a subject or an object, just as we’d use “Queen Victoria,” “Catherine the Great,” or “Aunt Hilda.”
George Bernard Shaw, for example, uses it as an object in his 1911 play Getting Married. When asked whether he’s staying for breakfast, Hotchkiss replies: “How do I know? Is my destiny any longer in my own hands? Go: ask She Who Must Be Obeyed.”
The Oxford English Dictionary describes “She-Who-Must-Be-Obeyed” (it uses hyphens) as a colloquial, usually mildly depreciative noun for “a strong-willed or domineering woman, esp. a wife or female partner.”
The earliest example in the OED (from H. Rider Haggard’s 1887 novel She) uses the noun phrase as a subject. Here it refers to a powerful queen: “ ‘She-who-must-be-obeyed’ commands thy presence, my Baboon.”
Oxford also cites a TV script by John Mortimer, who uses it as an object in an episode of Rumpole of the Bailey that was aired in 1978, the year the British series had its debut.
In the script, Horace Rumpole says, “Hoping to turn a bob or two which won’t be immediately grabbed by the taxman, or my clerk Henry, or by She Who Must Be Obeyed.”
(Hilda Rumpole, the barrister’s wife, is often referred to as “She Who Must Be Obeyed,” not only throughout Mortimer’s TV scripts, but in the short stories and books that followed.)
The noun phrase is also an object in the most recent OED citation, from the Nov. 18, 2007, issue of the Sunday Mail in Brisbane, Australia:
“The groom [was] wearing his future mother-in-law’s corsage. He had picked up the flowers but didn’t realise the beautiful buttonhole was meant for she-who-must-be-obeyed.” (In British English, a “buttonhole” can be a “boutonnière.”)