Welcome, JHawaii!

Hmmmm. I'm in disgreement with the majority.

He is a good friend of Mary. He is good friend (let's replace Mary with a pronoun here) of her. That flat doesn't ring right to me. He is a good friend of her. Nope. Don't like it at all.

He is a good friend of Mary's. He is a good friend of hers. Now that sounds right.

I'd go with: He is a good friend of Robert's. He is good friend of his. Not: He is a good friend of Robert. He is a good friend of him.