The problem is that employers like this pray on people trying to come to Japan, and who agree to land without a visa. While some of the claims, such as sick leave, have no legal validity, the teacher was indeed owed the money for working. Despite not having a visa.

The way to resolve this would have been to lodge a complaint to the Labor Standards Office before leaving Japan. or to appoint a third party in Japan who could take action for you while you are outside of Japan. The problem though is as you aren't in Japan, the LSO may well just take the side of the employer.

As hard as it is to hear I think you should just take it as a life lesson and move on. BUT publicize the case as much as you can to forewarn others from falling for her scam. And a scam it is.