My Coworker has been paid their “training rate” for the last year and a half instead of regular pay. What are the best resources for getting them what’s owed?

For context she is in the United States. We’ve both been discussing our pay and raises and I go over her pay stub and realize she’s been making $4 an hour less than promised in her onboarding. The company is only willing to backdate 2 weeks worth of her promised rate. How should she go about resolving this and what is the best place to start?