You don't need any evidence or printed documentation showing that it is illegal... you only need to show where it was not specified in her Employment Agreements, Handbook, etc. Originally signed upon starting there.
If they included it in documentation that she signed, then they have legal warrant to do what they are doing. However if it is not in any documentation that she has signed agreement to, then it is Illegal, and the verification for this would be their lack of evidentiary documentation.
Also, if THEY ARE legally doing it, they would need to issue her receipt upon request, so she may claim it for tax reasons, just as she must claim her tips and other wages.