'They' with singular reference

The third person plural pronouns ‘they’ and ‘them’ and the related possessive adjective ‘their’ are sometimes used to refer to a single person of indeterminate gender. When used in this way, ‘they’ is equivalent to ‘he or she’, ‘them’ to ‘him or her’, and ‘their’ to ‘his or her’. For example, 'The person who is appointed will teach parts of the department’s undergraduate programme; they will also be expected to undertake research in their specialist area of expertise'; 'A new teacher will arrive tomorrow: we look forward to welcoming them to the school'.

This use of ‘they’, ‘them’ and ‘their’ to refer to a single person who may be either male or female is especially common after indefinite pronouns such as ‘anybody’, ‘nobody’, ‘someone’ etc., and such phrases as ‘the person who’ or ‘a person who ‘ For example, 'Nobody enjoys having their work criticised'; 'If anybody would like to borrow this book, they only have to ask me for it'.

This use of ‘they’, ‘them’, etc. is useful, and AWE recommends it as a way of avoiding sometimes inelegant constructions of the form ‘he or she’, etc.

Rather differently, ‘they’, ‘them’, and ‘their’ are also sometimes used, though only in informal speech, to refer to a single person whose gender is already determined, but who has not been named or fully identified. On this use ‘they’ is not a substitute for ‘he or she’ but, according to the context, either for ‘he’ or for ‘she’, while ‘them’ is a substitute either for ‘him’ or for ‘her’, and ‘their’ is a substitute either for ‘his’ or for ‘her’. For example, 'Yesterday I met a man who asked me if it was far to the station. I told them the way and saw them set off in the right direction'; 'There is only one male doctor in that practice, but I think they are the best'; 'Who was the woman wearing the green coat? I need to contact them because they’ve left their briefcase behind'; 'That is the man I saw yesterday. I wonder who they are and where I’ve seen them before'.

It is hard to define when native speakers find this way of speaking acceptable and it is quite possible that there would be disagreement among them on the issue. However, it seems safe to say that the briefer and less informative the description of the individual to whom reference is made, the greater the likelihood that the use of ‘they’, ‘them’, or ‘their’ to refer to him or her will be found acceptable.

For some more examples see Michael Swan, Practical English Usage, (2nd edition, Oxford, 1995), sect. 505.

See also Number words - with pronouns.