Why is it correct to use the expression "He is a friend of Harry's." instead of "He is a friend of Harry." or "He is Harry's friend."? Convention seems to dictate that the first sentence is correct, even though "Harry's", an object of the preposition "of", is a possessive ?adjective and not a noun. He is a friend of Harry's what? Is this not grammatically incorrect?
Itís correct to say "He is a friend of Harry's." This is called the double possessive, double genitive, independent genitive or appositional of-phrase. Itís an old and well-established idiom. Itís equivalent to he is one of Harryís friends. It is indefinite. You wonít say he is a friend of *me or *you, you say he is a friend of mine or yours; why would you say "He is a friend of Harry?" Harryís friend is fine. You mean a particular one. "Harry's" is a possessive pronoun.
Itís grammatically incorrect or doesnít make sense to ask "He is a friend of Harry's what?"