A friend of Jack's - Tradução em português

When he arrived, Will saw a friend of Jack's; a woman with red hair and a red dress.

Por que utilizaram 's após Jack? Eu teria achado compreensível "a Jack's friend" ou "a friend of Jack".

Fonte: Pirates of the Caribbean - Dead man's chest. Pearson Education. Level 3. Pg. 7.

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2 22 127
It's a very old usage, and it sounds completely natural to me, even if it looks weird.

This was referenced in that thread Paulo linked, and if you are a super grammar nerd or a philologist it's a good read: tandfonline
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4 38 596
Por mera informalidade! Eu penso que trata-se de uma transcrição -roteiro do filme > livro, (não estou certo disso). Fato é, o modo como foi escrito neste trecho não está incorreto, apenas mais coloquial.
6 49 1.3k
We can say a friend of mine (um dos meus amigos/uma das minhas amigas). Though we can translate it as "um amigo meu" etc, but in that sense of the translation in parenthesis.
So, "one teacher of mine/a friend of hers/his, etc.

A friend of Jack's can be rephrased as One of the friends of Jack (that is, the structure is an indication that Jack has more than one friend).
It's usual in conversations between two students to say "a teacher of mine", he/she is talking about a particular teacher (of many) that stands out in his memory for some reason (at the moment of speaking).

So, the sentence is perfectly okay. Perhaps the quirks of the English grammar or somehow it goes against the logical reasoning of the Portuguese language. It's normal to the learner to find it odd, at first.

You also might like to read the following:
Ref. forum.wordreference
Então, seria uma estrutura estranha gramaticalmente, mas usada normalmente?

Do link: "In B/E it is correct to say "a friend of his" and "a friend of Peter's", though I agree that it is illogical."
6 49 1.3k
There are differences in usage between British English and its American counterpart. Perhaps the comment there chalks it up to this, I am not sure right now.

In my experience "a friend of his" is standard English form, and there's a Collins book - Commons Mistakes in English - by Timothy Fitikides
That points out that is an error the usage of an object pronoun in a double genitive. Example given:
Don't say: A friend of him told us the news,
Say: A friend of his told us the news.

Maybe I have read too much Bee Bee (BBC), The Guardian, and British novels! But it's normal to me. Perhaps one American would prefer "one of his friends." That's fine by me, too. Granted, it seems phonetically a bit baffling and off-putting to new learners, but less so when you get used to it.
Others may think otherwise, and I am okay with that as well.
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Um raro material de 1950, explicando a construção do "A Friend of mine" :O
6 49 1.3k
Some things are timeless! ;-)