I'd be conflicted too. If I'm in their position I wouldn't want to be reported. I recommend looking for resources that you can give to these people so they can escape from that hellhole.
A slap on the wrist for the business owner and legal trouble for the immigrants does not sound like justice to me. If you'd like to handle it responsibly, find a local immigrant refugee community organization that support illegal immigrants, and talk about it to them. Perhaps you could help set up some Asylum situation, and still report the guy while helping the workers at the same time.