It sounds like they are hoping he will not fight them. (Or else something else entirely is going on.) My personal take is that they are severely questioning his integrity, and have indeed slandered him. Counter with an offer (presented in a letter from a lawyer) where they acknowledge the accuracy of his "resume, qualifications, education, contacts, everything" and offer a severance package that will last long enough to find replacement work at a salary commensurate to his education, background and job experience. This may or may not be what he was paid before this situation came about. They have no reason to prevent him recieving unemployment as that was already paid. If they want him off the books, this would get him off of them. It also presents him in a better light to future employees than if he is severely demoted.
Right to work states are typically about union vs non-union employment, not about trumped up claims of invalid credentials. Something stinks, and I think the legal advise you have already received is probably not hte best available. Being fired for no reason is allowed, stating that your integrity and credentials are lacking is something else entirely.