If GA is an Employee At Will state you can terminate for any reason. If they are using company computers for personal use they have no real expectation of privacy. Worse, if they are trying to steal your clients they dont deserve any.
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You have strong rights here. I'd suggest that you FIRST covertly find out what else is going on with those on line chats BEFORE you approach the employee. You will learn who is at the other end of those conversations. If you take legal action, you will know who you need to depose. The more you know, the more power you have in litigation. As an employer, you have the legal right to install monitoring software which captures info in the background. Install something like webwatcher.com to capture info for a few days.
You need to see a computer security tech confidentiality and have monitoring software installed done it is YOUR computer and YOUR intellectual property this crook is likely to be stealing. You also need to watch your cash drawer, your tools, and your customer lists and account books. As soon as you nab a violation, you likely need to can this guy and do any necessary damage control. And, let this be a lesson to verify even when you trust.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.