If your girlfriend works for McDonald's, she is an at will employee and the employer can reduce hours, cut hours, change schedule, change pay, etc. at their own discretion. She is not being singled out because she is in a protected class but is part of an entire group of employees being targeted so there may not be much she can do. As for other labor code and corporate violations that are vaguely referenced, she should talk to an employment attorney to determine if any of the claims have merit.
It is not illegal for an employer to reduce the hours an employee may work. And while it is illegal for an employer to hire and retain undocumented workers, I fail to see how your girlfriend has standing to file any legal claim unless she is the victim of unlawful discrimination because of her documented status. The employer could get in trouble for employing undocumented workers but this will not benefit your girlfriend in a monetary way.
You mention other labor code violations. If the employer is, indeed, violating laws which adversely affect your girlfriend, she should consult with an employment law attorney to discuss her legal rights and options.
They say you get what you pay for, and this response is free, so take it for what it is worth. This is my opinion based on very limited information. My opinion should not be taken as legal advice. For true advice, we would require a confidential consultation where I would ask you questions and get your complete story. This is a public forum, so remember, nothing here is confidential. Nor am I your attorney. I do not know who you are and you have not hired me to provide any legal service. To do so would require us to meet and sign written retainer agreement. My responses are intended for general information only.
Depends on what these "many labor code and corporate violations" are. A full consultation is necessary to properly advise you.