Here is a thread with excellent pictures of canker, and a link to read about the disease: https://www.backyardchickens.com/t/27193/whats-wrong-with-this-chicken
http://www.thepoultrysite.com/diseaseinfo/154/trichomoniasis-canker-frounce
I would have to agree that that looks like canker, at least from the pictures I have seen of it. You really need to take this chicken back--don't ask for your money, but take her back where this lady has a flock that is infectious. If she were your own that had been in the flock, I would tell...
She could have been picked on because she has a contagious disease that will infect your flock. Sick chickens don't feel well, and are pecked. You should take this chicken back, or cull her. She is sick with either a respiratory other other disease such as MG (CRD), coryza, canker, or maybe...