The general rule there is to just try to make it a plural construction if at all possible, thereby making the gender-neutral "theirs" correct. Which is absurd. We shouldn't force ourselves into plural construction because we have no viable options. I vote to include "theirs" as proper.
I should also point out that people often make mistakes when speaking and writing informally -- and to point those out is probably a tad ugly. Now, if you're mentioning to a student or coaching someone to speak formally, then by all means. But, we should make allowances for the vernacular.
"Fundamentalism means never having to say 'I'm wrong.'"
-- unknown
Updated On: 10/31/07 at 05:00 PM