Can an employer disclose private information viewed on a company computer?

Question Details:

I had some private chats with a co-worker on a company computer (stupid of us). We discussed other coworkers and when my boss chose to review the screenshots and saw the chats, he decided to tell the other employees what we had said about them. Furthermore, he threatened to tell my co-worker's wife about something he had said (didn't specify what), saying "Oh, she'll care!" My friend says this is harassment. Clearly we shouldn't have shared personal stuff on the computer. Does that mean my boss has carte blanche to reshare that private information? What if it had been a private medical result? Important Notice