19She's the other woman
This takes us back to the second episode of season one “Showmance”. Rachel and Finn (Cory Monteith) have a special rehearsal in the auditorium, during which Rachel and Finn kiss. At the time, however, Finn is dating Quinn (Dianna Agron) which means that he technically cheated on her.
Maybe, like Rachel
said in the pilot, she was trying to push the two of them together as the potential “stars” for the club. Still, however, given how naïve she can act at times, this seems like a rather calculated move designed to hurt her bully.