Can a employer make you go home early?

My girlfriend (which is 16) had a shift from 4–10 at Starbucks and the manager made her go home 2 hours early and said that labor laws say she can not work for more than 4 hours a day on a school night. (it’s friday) I live in Illinois. If you know any reputable sources where I can find information on this so we can show her manager, that would be great.