The situation facing your friends seems to be a lot like those where the managers responsible for resolving the situation are in fact those doing the bullying. While it is possible to seek redress by following procedure and approaching the bully's superior, it may not work and can make the situation even worse for the recipient of the bullying.
In addition, where the bullying is non-physical, the emotions involved can make it difficult to provide specific details about has been done to make them feel the way they do. Even if children can provide precise details of what has been done, it is very much the teacher's word against theirs.
There seems to be very little parents can do, though in one case recently in Ohio, the parents of a 14 year old girl with special needs managed to obtain evidence to support their daughter's version of events (you will need to Google for the link). Despite very clear evidence of professional misconduct on the part of the teacher, she remained in post. Given the circumstances you describe, it seems unlikely that the outcome in your friends case would be any different, even if there was proof of unacceptable conduct.
It is a sad indictment of the system that there is still huge scope for teachers to have a very negative effect on those for whom they are responsible, and there is little that parents can do to mitigate this. Hopefully a move to another school is not out of the question.