can i sue a previous employer

Your wife can sue those who made the false statements for slander and/or libel (depending on whether the false accusations were written or oral).  Whether she can successfully sue the owners of the business (I am assuming that the business is not a separate legal entity; if, for example, the business is incorporated, the corporation may be liable for the actions of its employees, but the individual shareholders of the corporation would not be, although they could be liable for their own actions if they participated in the defamation themselves)  is a more difficult issue.  Assuming they did not participate in the defamation themselves, by either spreading the lies or instructing their employees to do so, they could still possibly be found liable if the wrongful acts of the employees were done in the course and scope of their employment.  For example, if the employees defamed you in an attempt to gain a competitive edge for their employer, that would be in the scope of their employment, and the employer would likely be responsible.  If that same employee gossiped about your wife to his brother-in-law, having nothing to do with the business, the employer would probably not be liable.