Please would a moderator comment. There is a massive difference between English and French. One can say 'he eats his cheese' in English. But 'il mange son fromage' does NOT tell you if it is HIS cheese or HER cheese. Son fromage simply refers to a masc noun and 3rd person singular. There is no particular context here. This is a really important point about French Grammar.
To specify "He is eating her cheese" you would need to use the disjunctive pronoun elle after the preposition à to indicate possession:
Il mange son fromage à elle → "He is eating her cheese".
Otherwise, you just take it as read that the subject is acting on their own behalf -i.e he is eating his cheese.
You can read more about disjunctive or stressed pronouns here:
Because while "son" (and "sa") may grammatically be used as with "his" or "her", each will be understood to refer to the subject of the sentence. Therefore, "il mange son fromage" will be understood as "he eats his cheese". If you want to be clear, you can say "il mange son fromage à elle" = he eats her cheese.
i was going to write, he eats her cheese, but that sounded a bit lewd, so i put he eats his cheese, and got it right, but yeah how can we differentiate if the words are the same for masculine and feminine? I scrolled down and couldnt find a sufficient answer for this conundrum