Okay so you guys are really confused on this situation so I'll try to explain it the best I can.
Hailey fainted the last time she was on stage, meaning she never went to the concert with Jacob. The concert and all that after it was all a dream. None of it ever actually happened.
She was never dating Calum or Kian, she never formed a band with Cody and Ethan and Jared, she never met Lia or Kenzie (although Kenzie is still going to be in the book as Hailey's best friend and Trevor's girlfriend).
I hope this cleared a lot of stuff up