I'm no legal professional; This would be reach out to an org (they may have more ideas) and ideally lawyer as well time, afaik you can't file anonymously without some loops in specific cases, best the lawyer can do is get some anti-retaliation paperwork going. It'd probably be best if your friend and their coworkers split legal costs and approached legal help together, if possible.

Are they unionized? Unions won’t initiate proceedings without your express consent, so if they have one on their side, they can talk about the situation and resolution. Not sure how you would resolve this solo, any of my thought would involve identification

I realize this isn't what you asked, but they should contact some lawyers who do labor law. Government agencies are generally hard to motivate even when you give them proof. Anonymous claims that require investigation aren't likely to be pursued.

Even though 10-15 minutes a shift might seem like not enough money to incentivise a lawyer, they can file a lawsuit on behalf of all similarly situated employees for all of the lost wages. Even if your friend doesn't want to be identified in a lawsuit, they can get a consultation. The lawyer can, especially with help from your friend, find someone to represent the group(like someone who no longer works there).