Are you a friend of John or a friend of John’s? Often writers will leave off the “possessive” apostrophe+s, but you need it. Think about these two phrases:
A portrait of King Henry
A portrait of King Henry’s
In the first instance, you have a portrait of the king. In the second instance, the king owns a portrait. There’s a huge difference in meaning here. So are you a friend of John? No, you are John’s friend—a friend of John’s. And hopefully, he is a friend of yours (not a friend of you).