In fact, most people who came across such advice would figure that: 1. The advice-giver must know what he’s talking about, and 2. Simple logic supports the claim. And if this advice were published in a scholarly professional journal, that would seal it for most people. Clearly, it must be true that you can’t say “a friend of John’s.”
No doubt, that’s what a lot of readers of the January 1996 edition of the journal “The Practical Lawyer” assumed when they came across an article saying exactly this.
That’s how myths get started. Just because someone has the credentials to get published in a journal or a magazine — or even a book — doesn’t mean he’s qualified to lay down rules.
“Some people erroneously stigmatize ‘a friend of mine’ or ‘an acquaintance of John’s,’ in which both an ‘of’ and a possessive form appear,” writes Bryan Garner in “Garner’s Modern American Usage.” In fact, he continues, “Using both the s- and of-genitives together is an English idiom of long and respectable standing.”
You don’t have to think about this for long to see the logic. Imagine, for example, that instead of talking about “a friend of John,” you wanted to talk about a friend of the person you’re speaking to. A standard way to put it is “a friend of yours,” which uses both the possessive “of” and the possessive “yours.”
If opponents of the double possessive were right, you’d have to replace the “yours” with “you,” which would leave you with “a friend of you.” There’s a reason no one uses that construction. It’s unidiomatic, unlike “a friend of yours,” which is standard.