Why does he use 's and say "friend of Rachel’s", not "friend of Rachel"? I know that I should put 's when I'm saying "Rachel's friend".
Native speaker’s answer
Rebecca
He says "friend of Rachel's" because something used to follow after the person's name (here, "Rachel") that is now omitted. In modern English, it is not known what used to follow after someone's name in this phrase, so it does not make sense to use " 's" here. Therefore, "friend of Rachel", or better, "Rachel's friend", should be used. However, both "friend of Rachel's" and "friend of Rachel" are considered grammatically correct.