Firstly, this is a special, not a general question about mobbing (bullying). My mother was without a workplace for a long time, now after two years doing nothing she finally found a place to work at a facility as a production operator in a almost women-only production. Most of them are even same age like her. She did make some mistakes in the beginning, and since then, two more experiencing colleagues are mobbing her.
They communicate very aggressively with her, the shout at each small mistake, talk very bad about her whenever she is around (probably even when she is not around), and they never talk to her in other occasions. During lunch break, every prod operator is smoking but so since she quitted smoking long time ago she is never joining them. My mother says she hardly makes any mistakes now since one month but is still threated very badly. Other new employees are treated much better according to her. She talked to her boss but it seems like he is constantly getting information from the more experienced bully-colleagues that my mother is a bad employee so he did not help her. Past shift when she was alone and not being harrassed by the bullies she said her performance was flawless and she got even compliments from other colleagues. Then, once she was no longer alone, her colleagues continued harrassing her by shouting at her and constantly criticizing her and she started to make more mistakes according to her.
So - what to do? I ran out of ideas how to solve the situation. I also want to add that my mother has academical degree and always worked white collar, never experiencing working with the harsh reality of low educated, unskilled people like some production operators are.