People are increasingly using the plural pronoun they to refer to one person if they do not know whether that person is male or female. Until quite recently, he was generally used to refer to a person of either sex, as in 'Every child needs to know that he is loved', but nowadays many people feel that such a use is sexist.
He or she is possible, but is rather awkward. They is generally accepted in sentences using words such as someone or anyone, e.g. 'Anyone can join if they are a resident'. More people object to they being used after a single noun, as in 'Ask
a friend if they can help'.
Interestingly, use of they in this way is not a modern invention: it was first recorded in the 16th century.
