Q. I'm involved in a dispute with one of our celebrity authors (name withheld). We contend that it's now acceptable to use "they" and "their" and "them" to refer to singular antecedents, such as in the following sentence:
If your partner is cheating on you, they will reap the consequences.
Do you agree that this is now okay?
I need a specific grammatical rule from a source to show him.
A. I agree with your author. Although the kind of sentence you give in your example is heard more and more frequently, it still sounds wrong to me. The Gregg Reference Manual calls it wrong and gives various strategies to avoid it. Keys for Writers by Ann Raimes and A Writer's Reference by Diana Hacker, both published in 1999 consider that usage incorrect.