Rav,

I'm not sure what you don't understand, but let me list here some examples so you might understand my viewpoint better.

First, we can replace nouns and proper nouns with pronouns. Pronouns come in handy when figuring out points of grammar.

I'll list below some examples:

1. Jackie and AnnaS posted only three times in one week on the board.

2. Jackie and she (not her) posted only three times...

3. They posted only three times...

4.. I read two posts of Jackie's.

4. I read two posts of hers (but not: her).

I wouldn't write:

5. I read three posts of Jackie.
Or:
6. I read three posts of her.


Now back to Robert and his friends.

We can get at this idea many ways:

1. Who is Robert's friend?
2. Who is his?

They both work.

1. John is a good friend of Robert.
Now we've got a problem if we want to replace the object of the preposition with a pronoun. Strictly speaking, Robert must be replaced with third person singular objective case: him. So we end up with:
2. John is a good friend of him.
That doesn't work for my ear.

Even worse, suppose I replace Robert with myself:

1. John is my good friend. John is a good friend of mine. (2 examples that work.)
2. (But this doesn't work:) John is a good friend of me.
Yuck!

So, now take a look at the original sentence offered by John Hawaii:

1. He is a good friend of Robert's.

Like Modestgoddess, I think that sentence is perfectly grammatical. Test it out with a replacement by a pronoun and you get:

1. He is a good friend of his.

That works, too, grammatically. Where I have a problem is when we replace "Robert" with him--He is a good friend of Robert...He is a good friend of him. That doesn't work. You would be better off having: He is a good friend of Robert's; and replacing 'Robert's' with 'his' in He is a good friend of his. Or a friend of yours. Or a friend of hers. Or a friend of mine.

But the sentence is a tricky one and I can see where the others are coming from. If I were to read: He is a good friend of Robert--I'd understand, wouldn't parse, would just keep on trucking at a steady beat.

This is just my opinion. It's just mine.

Best regards,
WW