In a qui tam action, the private party helping the government is called a relator. If the prosecution succeeds, the private party receives a portion of the reward. See False Claims Act.
For example, if an employee of a defense contractor discovered that his boss was defrauding the government, the employee could file a False Claims Act qui tam suit against his employer. In that action, the employee would be the relator, the employer would be the defendant, and the government would be the plaintiff.
See, e.g., United States ex rel Eisenstein v. City of New York (08-660).