The problem with making a legal case is that she doesn’t have proof she did actually work those hours, she would need to get coworkers involved as witnesses which they would most likely be wary of doing in case it affects their employment…maybe she has clients she can contact as they would have nothing to lose by giving a statement? Request any CCTV?
It might be a tricky one and it sucks big time, she needs to gtfo out of such a shitty job and maybe just cut her losses while bashing them on social media every chance she can get, the backlash might prompt a response from the company.