What can you do if you are being treated unfairly by your bosses at work, and even people higher up on corporate ladder?

First off, I'd say that your friend should be seeking the advice if this is important to her.  Without knowing more, I'd say probably nothing she can do. There's no right to be given the same amount of work as others.  Maybe there's some actionable discrimination here (based on race, gender, religious affiliaton) but usually that's not the case and even it were, its difficult to address.