Thank you for using JustAnswer.
I'm deeply sorry to hear about your situation. First of all, you need to understand that both Hawaii and Georgia are "at will" employment states. At-will employment means that without a contract, you have no contractual or other right to employment with the company. The company is entitled to fire you for any reason: a good reason, a poor reason, or no reason at all--as long as the company does not fire you for an illegal reason (race, gender, age, religion, etc...). But it extends beyond firing, to hiring, promotions, demotions, wage cuts and raises, disciplinary actions, and even scheduling. Unless you can show that this was done in violation of a contract, union agreement, or a clear violation of an unambiguous and binding clause against the employer, or that it was done because of some minority status (age, race, gender, religion, disability) that you have, then they do have this discretion.
So in short, it does not appear that there is a case for wrongful termination, and there was no obligation to inform him of the pending investigation. Yes, you should acquire the handbook to see if there were any internal disciplinary procedures that were not handled, but even then only if you could show that there was a guarantee that said procedures would be followed and that IF followed, they would have necessarily resulted in a different outcome would you have any chance of success under a "quasi contract" theory in court. Otherwise (and I'm very sorry to say this) you wouldn't have a case.
I know this is probably not what you wanted to hear, but it is the law. I hope that clears things up anyway. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable. Please note that I don't get any credit for the time and effort that I spent on this answer unless and until you rate it positively (good or better). Thank you, ***** ***** luck to you!