Legally, it's a grey area. If your employment contract has morality clauses, for example, you can be punished for things done outside of work. However, usually that is limited to situations where your contract explicitly states it, which usually happens when working for religious institutions (or, occasionally, schools). You can also be fired for actions that reflect badly on your company, but that assumes that A. people know the author works for that company, and B. they have reason to somehow connect the two. And of course, in at-will states, your employment can be potentially terminated for any reason, though in many, the implied covenant of good faith might give the author grounds to argue that this was without cause, done out of malice arising out of personal embarrassment on the part of the management team.
The bottom line would be that the author should contact a lawyer who regularly deals with employment law in that part of the country, because whether he has a case or not is highly dependent on where the author is located, and I'm pretty sure it won't be open-and-shut no matter where the author lives. However, the fact that the author has not revealed where he works does open the opportunity for the lawyer to point out that bringing this to court will cast their company in a very bad light publicly, whereas an out-of-court settlement for... say ten years' salary will not. Depending on how terrified the company is, such (entirely legal) blackmail might actually be more effective than bringing a suit.