I'm a friend of Stephen's.
It's British English, period: around 1970's.
Do we need that 's at the end of the name? I can see why it is there when you say, for instance, "I was at Stephen's." Here you omit something; you can put something after Stephens's and complete the sentence: "I was at Stephen's home."
But in the first sentence you cannot put anything after Stephen's.