It's against the law for employers to tell, or threaten to tell, the government about an employee's immigration status because the employee stood up for their rights. Some of these rights include speaking up about not being paid properly, unsafe work conditions, or sexual harassment. This rule applies to job applicants, current employees, past employees, and their families.
Also, usually, information about someone's immigration status can't be talked about in court and usually can't be used as evidence unless it's really needed to prove something or if it's important for an employer's defense. For instance, it might matter if an employee says they were treated unfairly because of their immigration status. But usually, immigration status wouldn't be something talked about or used in court if an employee is saying they weren't paid properly by their boss.
(See Link: Evidence Code Section 351.4 and Labor Code Section 244)