When referring to a single person of unknown gender, is it correct to use le/la leur, or is that a situation where theirs should be translated as le sien/la sienne?
E.g Would "Someone has forgotten their wallet. Don't take it! It's theirs." translate to "Quelqu'un a oublié leur portefeuille. Ne le prends pas! C'est le leur." ou "Quelqu'un a oublié son portefeuille. Ne le prends pas! C'est le sien."