He is John's friend = He is a friend of John's He is my friend = He is a friend of mine He is their friend = He is a friend of theirs. He knows them.

A friend of theirs :) Because you say a friend of mine. It shows possession, like, my friend. Hope that helps! :)

This is an important grammar point. Possessive pronouns (my, your etc) have two forms. One when they go before the noun (my friend) and one when they stand alone or go at the end of a phrase (a friend of mine). They are: my friend your friend his friend her friend our friend their friend and a friend of mine a friend of yours a friend of his a friend of hers a friend of ours a friend of theirs Learn them and remember them, and you'll never make this mistake again!

You also would say: He's a friend of his. He's a friend of hers. You would not say: He's a friend of him. He's a friend of she. There is used for places. Like let's go look over there.