Can THEY or THEIR be used as singular?
I understand seeing this question you can’t help smirking. Or might be scowling? Because, your understanding is not permitting you to accept this. However, you need to think.
Everybody should take care of their own luggage. ( Everybody is singular but corresponding pronoun cannot be his or her since this word is gender neutral).
Every driver should produce their license when they are asked to produce. ( Here the driver doesn’t mean male. )
So , when it is gender neutral They/ Their can be used for singular. Instead of this if you use HIS or Her you will be certainly termed as gender biased!