In the Glee 'Ballads' episode, it is actually Rachel who convinces herself that she and Mr. Schuester won't be together. She has developed a crush on him and confides in Finn about her feelings. Finn tells her that Mr. Schuester is already married and that their relationship would never work. Rachel agrees and decides to move on from her crush. However, there is no specific character in the episode who convinces Rachel that she and Mr. Schuester won't be together.