I've had extensive experience working with bullies in middle school and training peer counselors in an anti-bullying program as a teacher. I also work with quite a few adults as a therapist, who are victims of people who most likely have narcissistic personality disorder (and are even crueler than those women at the barre class). There is generally a lack of empathy on the part of people who display this type of behavior. They cannot begin to change this pattern and see the effects of their behavior until they are confronted with it. While Lauraelmore (or any victim) shouldn't have to "cure" these people, saying nothing will just perpetuate the cycle.
The gym owner definitely needs to address this issue, now that she is aware of it.