In "The One With The Sharks", Joey meets Hayley and asks her out (with some interruption from Gunther). On their date, upon returning to Hayley's apartment, Joey realizes that he has already had sex with her. He is extremely offended that she doesn't remember him, despite the fact that he didn't remember her.
Finally confronting her, Joey realizes that it was not her that he slept with, but her roommate. Unfortunately, he only realizes this because Hayley's roommate walks in.
Hayley's parents are divorced, after 25-years of marriage. Hayley says she took it the hardest. She has at least one older sibling.