What legal actions are available to use against an employer who blatantly steals from his employees?

Ok, so I know someone who is a single mother of three, very hardworking, devotes SO much of her life to her job. She works for straight commission as a salesperson, and recently she had a customer who was late on payments. Because of this, her boss took $750 out of her paycheck right before Christmas time. This is the way things work, if a customer doesn't pay then it comes out of the employees paycheck, it sucks, but that's the way it is. The messed up part is that the company that was late on payments paid the $750 in full the very next week. When the employee in question went to her boss and asked "Now that the company has paid, will I be reimbursed for the $750 you took out of my paycheck?" Her answer was "no". When she asked why, the response was "Because that's the way I have always done things." So in other words, the boss got paid twice for the same product order, once from the company that places the order and once from the employee who struggles to take care of her family. Where did this extra $750 go? I have no clue. I feel like there has to be SOME sort of legal action that can be taken, because this is not the first time her boss has done this to an employee, and probably won't be the last.