What can be done if a former manager blacklists you?

A friend of mine was antagonize into quitting her job. There were long standing issues between them that started almost as soon as the new manager started working there.

She understood that there would be problems collecting EI, but after being told to "get on her knees, and scrub like Cinderella " she walked out. She was told by this manager that she was going to blacklist her, and she would never get another job in the city.

It has been almost 2 years since then, and she has been unable to find a job because she has been blacklisted by her former manager. Twice she was hired by managers who were unaware of her prior issue with her manager, and was let go as soon as her ex manager found out she had a new job, and contacted her new workplace. Both times she was treated like a valuable new employee, and was then told that they simply didn't have hours for her anymore. She is now unable to even get an interview anywhere, even in a different industry.

The only reason she quit was because of the personal conflict. She had no write ups, or disciplinary actions taken for any reason.

Is there any legal action she can take against her former manager? If there is, how can she prove the malicious actions of her former manager?