In this case it can't be his. As far as I know "sin/sit" are like "his own/her own". It lets you know that the object belongs to the same person who is saying the sentence.
So here, the person is hun (she), and she is eating her own apple. so:
- Hun spiser sit æble > She eats her (own) apple.
She eats his apple would be Hun spiser hans æble
by the way, I'm not completely sure, so if someone can correcte, please do.
CAN YOU PLEASE COMMENT!!! some one gave me a note that sin/sit/sine can never be part of the 'subject' Therefore 1. Maria og hendes mand spiser sammen[Maria spiser sammen med hendes mand] (he can be her husband or another woman's husband) -Maria and her husband eat together//Maria eat with her husband. both statements are true , he said 2,Maria spiser sammen med SIN mand(here we are sure that he is maria's husband; and not part of the Subject so we used 'sin') but, he said: 3 ''Maria og SIN mand spiser sammen'' is false statement even if we are sure that he is maria's husband; because 'sin' cannot be part of the subject.(Sin/Sit/Sine refers to the subject but should not be part of the subject)