Actually, "He is friends with John" isn't something I'd use formally. You certainly hear it commonly used in conversation but, as Matching Mole said "He is a friend of John's" is more correct. You could also say "He is friendly with John" although this would be less commonly said.Actually neither one is correct. "He is friends with John" or "He is one of John's friends" are better.
Question: Who is this person?
No, I disagree. You are using the apostrophe, which implies ownership; but you are also using the preposition "of." This is a double-positive usage, which is excessive. I do not think excessive, double-positive usage is proper grammar. I don't know the correct termionlogy for this predicament, but I would not use that sentence, Matching Mole.Or: "He is a friend of John's".