This is the first page from a fiction novel, please read this text and then read my question at the end :
The girl checked the time oh her pink wristwatch and looked back at the front door of the school. Tracy, her best friend, should have been here by now. The girl sat down one the stone embankment and swung her legs hard, hitting the backs of her calves against the rock in a steady rhythm. No. She was wrong. Tracy wasn’t her best friend. She used to be. Then Tracy got to be friends with the popular girls.
"Then Tracy got to be friends with the popular girls."
Why the word friend is in plural and not in singular form??
thank you.