idioms - he became firm friend or friends?

I was reading BBC culture page ( See Here ). In the middle of the text, we see :

He became firm friends with the Shakespeare and Company owner in a relationship that lasted until his death 40 years later.

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Q1: Does the writer make mistake ?

Q2: Why the writer have used plural form of "Friend" ?

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Thanks

Answer

To be friends with, to become friends with, or to make friends with are idiomatic phrases. If somebody is your friend, you can say you are friends with him. If you want to start a friendly relationship with a person or persons, you can say that you want to become friends or make friends with him or them accordingly.

So the use of friends in the sentence presented by the writer is correct grammatically.