The fact is that a worker can be fired due to their appearance so long as it is not due to the inclusion in a legally "protected class". In other words, they can't be fired for their appearance it it relates to their race, sex, national origin, age (over 40), disability, etc. However, a company can terminate an employee on all other appearance based issues. Also, if there is a medical reason as to why your wife can't wear her teeth, she may have protection under the ADA (American's With Disabilities act). Otherwise, without an employment contract or union agreement to the contrary, she is an "at will" worker and can be discharged for any reason or no reason at all, with or without notice.