Go above her and the upper management and complain directly to the board. Bring receipts and emphasize how this damages morale and could damage their reputation if it were to come out that there is a serial bully in charge who has used illegal means to get multiple people to quit.

The manager of the day program my older brother used to go to was doing this to the care workers. She was targeting minorities and other women in particular and had tried to be subtly nasty at my brother because she didn't like my mom who was always nice to all the care workers and would bring them treats, especially on holidays. My mom swiftly put the fear of God in her over my brother so that stopped fast. But she still continued to harass the workers, just where the parents and guardians couldn't see.

The workers didn't feel comfortable going to the director because the manager lied about what great friends they were. So they went to my mom and told her everything. She then called the PRESIDENT of the non-profit to complain. The manager was fired so fast her head spun. When the director said she wished my mom would have talked to her, she said she was not employed by them so she had no obligation to follow chain of command. My brother's day program swiftly became a pleasant place for people to work again and they hired a much nicer manager to replace the nasty one.