老师们好，请问my grandfather had been corresponding with a little girl who was the daughter of some friends

为何这里用some friends这种复数呢？

摘录上下文，选自世界小史的英文版序言部分

My grandfather was not impressed by what he read: so little so that he told the publisher –Walter Neurath who later founded the publishing house Thames & Hudson in England – that it was probably not worth translating. ‘I think I could write a better one myself,’ he said. To which Neurath responded that he was welcome to submit a chapter.

It so happened that, in the final stages of writing his doctoral thesis, my grandfather had been corresponding with a little girl who was the daughter of some friends. She wanted to know what was keeping him so busy, and he enjoyed trying to explain his subject to her in ways she would understand. He was also, he said later, feeling a little impatient with academic writing, having waded through so much of it in the course of his studies, and was convinced that it should be perfectly possible to explain most things to an intelligent child without jargon or pompous language.