Now when you first get hired onto a job you are generally required to abide by a set of rules, most buisnesses make you sign the company rule handbook (or paper saying that you agree to follow that rulebook) before you start working now a days, your agreement to abide by those rules is a contract, be it a verbal contract or written. Now in this case I happen to know that BK does make you sign such a thing so there is probably evidence that she did sign it. And I would bet 100 dollars that no exceptions were noted down in that handbook which means she is still obligated to follow that rule book.
IMO the girl may have standing to sue the guy that hired her for something or other because he was in the wrong, but BK? Thier dress code for normal employee's is applied universally with no discrimination. They should not be sued for non-discrimination.