To show possession, we use whose for both people and things:
In the following examples, the man is the subject of both verbs and cannot be omitted:
In the following examples, the woman is the object of the verb 'saw' and, therefore the pronoun can be omitted:
NB - WHOM is an object pronoun. It is used in formal English after a preposition and can be used to replace an object,although many no longer do this. In a phrase like 'To whom it may concern', who would not be acceptable.