The words for “his” and “her” (and “its”) in French are son, sa, ses. However, the choice of whether we use son, sa or ses depends on whether the noun (the thing possessed) is masculine, feminine or plural:
Son frère = his/her/its brother (noun frère is masculine)
Sa mère = his/her/its mother (noun mère is feminine)
Ses yeux = his/her/its eyes (noun yeux is plural)
Can you spot the problem? If we want to make the distinction between his father and her father, how do we do it? Both are translated in French by son père!
One way of clearing up the ambiguity would be to use the name of the person whose father it is: le père de David = David’s father. Another way would be to add à lui (HIS) or à elle (HER), e.g.:
C’est son père à elle = It’s her father.
C’est son père à lui = It’s his father.
C’est son livre à lui qui manque = It is his book which is missing.