The use of the employer's computer and printer is a red herring. All it really brings to the conversation is that you are 100% sure he got an offer and accepted it, and that you're pretty sure management can't find this out for themselves. (You may be wrong: many firms log email including body and attachments, and the fact that it was deleted in a local mailbox means nothing.)
The illegality, should some be happening, is by the poaching company. If you became aware that members of the company you work for were breaking the law, and opening the company to risk, you might have an obligation to tell your management about it. Say someone tells you he prefers to drive home drunk in the company car instead of his own car, because if he gets in a crash at least his personal car will not be wrecked. You should tell someone about that. But this is more like learning that an employee of your competition likes to drive drunk. You don't need to tell your boss about it. If there's some third party you could tell, you might, but I would tread carefully with that (see below.)
What is left then? Well you might want to let your boss know that someone is behaving unfairly and causing him damage by stealing his people. But before you do, consider:
- are you sure he doesn't know? If you know, what are the chances he doesn't? It might be a little offensive for you to come and tell him this hot news, suggesting he didn't know.
- are you sure it's a problem for him? There's a relationship between the companies, one is doing better than the other, and people are switching sides: maybe the two companies agreed to this instead of a layoff? Maybe it wasn't his idea, but he doesn't mind, because it's reducing the salary burden during this low spot? Again, it's a bit presumptuous to imply that he needs your help on this matter. He knows things you don't.
If you're convinced that you must tell your CEO to protect the company, and that the company would be hurt if you didn't, consider setting up a throwaway email on gmail or the like, that you never access from a work computer or through work wifi on a personal device. Email the CEO and explain you want to stay anonymous but you wanted him to know what's going on. Don't mention non competes or notice or any of that - the CEO is perfectly capable of determining what notice was given and what agreements were signed. You might get a reply or you might not, but you won't have hurt your own prospects by embarrassing or offending the CEO with your "hot news."