Seems like such a waste of time to struggle around just using "their" in a singular meaning. I've been doing it forever and nobody seems to care. The above link down at the bottom has an interesting take on it. Can we use "their" in singular form for your class now?
I alternate between several things to say to this individual:
Which of these would you reply with?
The Associated Press officially announced this year that they are including the singular "they" in their style manual.
I realize the AP is journalism and not technical writing. That being said, the AP is a major content-producing organization. I think their announcement is an indication of the ever-changing standards.
I was taught to always put two spaces after a period. In my very first research job after grad school, I was told to stop putting two spaces after a period, that the standard had change. Things evolve in our lifetime.
In the linguistics column from the Economist earlier this year, the author points out that the first known use of the singular "they" appeared in 1375. It's been around for a long time. Not dominant, but present.
But obviously there are situations in which the singular "they" would not be appropriate. On the other hand, I think there are situations where insisting on never use it would make the writer come across as old-fashioned or tone-deaf. There's something to be said for adaptability.
It boils down to making sure students know the formal rules, know that standards change, and understand how to assess the audience/situation and make adjustments appropriately.
I agree with all that's been said re: the changing convention. In such cases I tell students the final ruling for such things resides within the disciplinary community. Engineers may not adhere to the new convention as readily as MLA does. Students need to learn rules are fluid and they should be prepared to learn and follow the styles and conventions of their audiences' disciplinary discourse conventions.
My response would be who is the audience? What is the context? If it is a formal piece of writing meant for an audience that expects it to be professionally written and edited, go with the rules as they are now.
He is asking if he can write as he speaks. Usually, we don't do that in this writing genre.
I'd go with your number 4 response given the context.
My first question would be, "Are they using "their" in a singular non-binary way?"
If that's the case, there is some legitimacy to the question. I attended a non-binary, transgender training at Texas State University (because I'm out-dated in my understanding of the issues surrounding non-binary gender related issues). We learned through that training that many students use "they/them/their" to refer to individuals who do not identify with a gender.
It's now something I consider when someone asks about gendered pronouns.
Some students use "ze" for non-binary singular expressions. I know that it's an issue for our Modern Languages faculty, who teach gendered languages.
Here's a little more insight into non-binary pronouns: http://www.bbc.com/news/magazine-34901704
Again, this student who emailed you might simply be lazy or hate the English language, but I wanted to mention other possible reasons behind asking about singular they/them/their.
I still worry about the precision issue. Here is a link to a story about an error involving restrictive/nonrestrictive usage that turned a big lawsuit:
Information provided by firstname.lastname@example.org and BGTCM faculty.