Dr. Mario said:
Not this "language evolves" argument again. It's true, but when there's still controversy surrounding it, I'd stay clear of it. Sometimes, you want to be as clear as possible, and sometimes, "they", feels cleaner than the blech-y "he or she", sometimes just doesn't cut it. Ultimately, though, that singular "they" still triggers heated arguments, it makes it less-than-ideal to incorporate it in writing. Just give it a few more decades and maybe it will be accepted everywhere.
Writing, especially formal writing changes way more slowly than spoken, so just because it's an "emergent vocabulary and novel derivational affixation" (really, now?) doesn't mean it's acceptable in writing. As Kurt Vonnegut Jr. said, "if it were only teachers who insisted that modern writers stay close to the literary styles of the paste, we might reasonably ignore them. But readers insist on the very same thing. They want our pages to look very much like the pages they have seen before."
I don't mind the "singular" they in conversation and informal writing; it flows a lot better than the clunky structures. I find in unacceptable in formal writing, though, and you will be dinged in English and grammar classes if you think the singular "they" is acceptable. There are dictionaries than contradict each other and themselves on the singular "they" and "they" isn't established quite yet in the language. There are quite lengthy arguments on the singular "they", so it isn't as clear-cut as what Walkazo asserts as "viable". To avoid such discussions, I'd rather not use "they" or "he or she" and rather reword the sentence as a whole (pluralize the subject, replace pronoun with an article, and other things). It's not a foolproof method, but it gets by a lot of times.
Singular "they" sounds awkward in this case either way. How would you address a known person whose gender cannot be identified? Like, somebody who is transgendered? "They" doesn't work. Neither does he or she.
Finally, I'd opt for "it". I don't care if it's dehumanizing, it's our speciest attitude that's blocking progress there.
So yeah, off-topic crap from a pedantic plumber person... thing.
Let's just agree to disagree here, m'kay