The exclusionary rule prohibits the government from using evidence illegally obtained by the police against the defendant in a court of law. I am not aware of any law that prohibits an employer from using illegally obtained evidence against an employee for disciplinary reasons.
The government is free to use evidence that was illegally obtained by a non-government agent against the defendant, so I doubt there is any law prohibiting an employer from using illegally obtained evidence in a disciplinary proceeding against an employee.
If there is a law that prohibits the cell phone company from using that information, the same law would describe what recourse, if any, the victim has.