I met with a friend of Molly's
I met with a friend of a friend
I met with a friend's friend
This just points that we are not yet completely comfortable with the fact that we have *almost dumped all vestiges of case structure in the English noun†. Originally we didn't much use prepositions, relying on case to tell us the relationship among nouns in a sentence. We now use word order and prepositions. One of the characteristics of the transitional phase was the requirement that the case of a noun was linked to the preposition. Since the only case we show by inflectional endings is the possessive (formerly known as the genitive) and the only preposition we use it with is of it is something of an anomaly and therefore raises questions. To take the example one by one:
A) I met with a friend of Molly's
This fits the discussion above. Given the meaning there really isn't any other way you can say it. You can't say a Molly's friend. The a here does have a meaning that can't easily be said any other way.
2) I met with a friend of a friend
Nothing wrong with this.
Þ) I met with a friend's friend
Just another way of saying 2)
†We are also in the process of losing it in pronouns, causing a great weeping, wailing and gnashing of teeth among the grammatically hypersensitive.
