"He is a friend of John's" is better. (Though I disagree with Louri - "He is John's friend" does not mean that he is John's only friend.)

Hello, 'He is a friend of John's' would be correct, or more naturally 'He is John's friend'. I hope that helps, Christine

He is a friend of John's. (One of his friends) He is John's friend. (Probably the only friend) "He is a friend of John" is incorrect.