Quando o sujeito é uma palavra terminada em one ou body , geralmente usamos they , them , their.

Everyone is having their lunch.

Everybody has arrived. Give them the books.

Porém se sabemos o sexo da pessoa usamos he ou she , his ou her.

The girls were playing volleyball. Someone has left her towel in the locker room.

Fonte : The new simplified grammar book (Amos / Prescher)