https://en.oxforddictionaries.com/us...he-versus-they
Some people object to the use of plural pronouns in this type of situation on the grounds that it’s ungrammatical. In fact, the use of plural pronouns to refer back to a singular subject isn’t new: it represents a revival of a practice dating from the 16th century. It’s increasingly common in current English and is now widely accepted both in speech and in writing.