I saw on one of my listservs that the Chicago Manual of Style has given up on “his or her” for the singular possessive. I know it requires re-writing to fix those awkward sentences: The user must check
their his or her manual for the exact procedure.
But languages have rules so that they are governed by logic that any speaker/reader/writer can follow to get the job of communication done. It is not hard to revise the sentence to achieve agreement: Users must check their manuals for the exact procedure. It is much more elegant to have a language that works because the more exceptions we incorporate the more vagueness we incorporate.
We count on reference sources like CMS to hold the line for the rest of us.
Tsk, tsk, CMS.