If the "religious quirk" is something that isn't going to prevent the employee from doing their job, or impair the job site's ability to function and do business, then making a MINOR accomodation to let the employee fulfill their deeply held beliefs is every bit as reasonable as letting an employee with foot problems wear orthotic shoes instead of company-issue shoes. To the person who holds such a belief, honoring the moral requirements of their religion is very important.
I have no problem with a Sihk or a Muslim wearing a turban while they hand me fries and a burger. I have no problem with a Jew handing me my change while wearing a yamulka. Why the hell is this woman wanting to wear a long skirt a big deal?