I understand how upset you and your daughter must both be that she lost her position on the cast of the school play. I don't know that you can categorize the incident as bullying, but it certainly seems upsetting that the authority to "hire" or "fire" someone was given to a fellow student. It raises the question of whether your daughter's ex-boyfriend is being biased against your daughter because she was the only one that was let go from the play. Every school and drama teacher has their own way of running a school production. It's not uncommon for a student to be given the authority to make casting decisions, however the instructor should always be there to back up the student and correct decisions made in error by the student.
 
You mentioned that the instructor is aware of the situation but you didn't specify if you have talked to that instructor yourself. If you haven't, I would strongly suggest that you call and make an appointment with the instructor to discuss what happened to your daughter. If you have done that but were not satisfied with the results, talk to the principal about the situation.
 
If the decision is irreversible, encourage your daughter to get involved in other things inside and outside of school. Remind her that she is a wonderfully talented person and that just because she won't be a part of that play, there are always more and better opportunities just around the corner.
 
Boys Town National Hotline
1-800-448-3000