My boss has fired my friend because he has read some emails on her personal email account (such as yahoo..) that we have been sending each other. We have been very frusstrated with his "management style" at a time and were saying not very flattering things about that. He has also found out through those emails that she's been interviewing for another job. He confronted us saying that he was just looking for something on her computer and her personal emails just "popped out" and he couldn't miss it when he saw his name. (My friend's computer was turned off all day untill he began using it).My question is: is it okay for him to invade our privacy by purposely searching through her computer (although he denies it) and reading our personal letters? Is my friend fired for no fault of her own? I will be quitting pretty soon myself, but I just want to know if there is a law that protects me agains that sort of thing? Was it illigal or just unethical of him to do that?
Thank you.

Your friend has no expectation of privacy if she used an office computer to send the e-mails. The employer has broken no laws by terminating her for using an office computer to do personal business, including both job hunting and complaining about the workplace.

Okay, so what if she was on a break (meaning clocked out) when she used the office computer to hunt for a job or to type those emails? Would that make any difference at all?
Also, if it's ok for my boss to open and read our emails in our personal email accounts, is it fine for me to come up to his desk one day and start opening his letters he receives? He uses a mail box that belongs to the company. Does he have any expectation of privicy?
Thanks.

No, it doesn't make any difference that she was using the computer on her break. It still belongs to the company. She still used the office network to access the internet and send those e-mails.

No, you can't just open mail on someone else's desk without repercussions. Among other things, that could be a violation of company policy but, in any event, receiving a personal letter at the office is a much more passive event than using the office computers to job hunt and slang the boss.