Why is it "a friend of his" not "a friend of him"?
The important fact here is the usage of "OF", the proposition used to establish the relationship between the subject and the object. So then the possessive pronoun has to be used to show that the subject belongs to the object. So Ross is saying I am his friend. It is important to note that "friend of him" is not grammatically correct. Ex: She was a friend of mine. Ex: Are you a friend of theirs? TIP: In some cases, you can change between the two forms to express different things. Ex: A picture of his – a picture he owns/possesses Ex: A picture of him – a picture that contains his image