I think the only correct answer is to talk to a lawyer and determine the best way to legally terminate the employee. Also, another lawyer should be involved to settle matters between the company owner and unfaithful wife. Things could reach another level of complicated if the wife decides she wants a say in the matter. Does she own any portion of the company, or have a voice in how it is operated?

The end action is obvious (to me). It's the legal minefield of steps necessary to get to that action that the owner needs to carefully consider (with the advice of sound legal council).