grammaticality - "A friend of Susan" vs. "a friend of Susan's"

I'm currently having hot debates with a friend of mine about which one is more natural and more grammatically correct:

• A friend of Susan


• A friend of Susan's

I vote for the first, but I don't have anything to prove I'm right.

Answer

A friend of Susan’s is a double genitive, which has been a feature of English grammar for centuries, and it is the normal alternative to one of Susan’s friends. Just as most people would say a friend of mine, rather than a friend of me, so a friend of Susan’s, rather than a friend of Susan, would be the natural choice in most contexts.