As others have said, way too many details missing for any informed opinion or analysis. Apparently, there is a State Identity Theft statute that they are applying to this case. The prosecution is very unusual to say the least and would set a terrible precedent given that suspicious men and women everywhere have snooped in their significant other's or spouse's email, texts, phones, mail, - you name it.
I do disagree with DB though on one point. Assuming "expectation of privacy" is even an issue here, and I do not think that it is, there certainly is an "expectation of privacy" as it concerns personal communication, including emails. Where you access that personal communication from is irrelevant. You can argue it's a shared computer in their shared household and that's irrelevant - the email account was apparently hers alone, and it was password protected.
If it were a shared email account, there would be no such expectation. But this was clearly (although it did not explicitly say so it's a safe assumption) a private email account - I mean, the woman was using it to communicate to a lover right?
Now that's I'm wrestling with this in my thoughts, I think there are some interesting questions to be raised here. Until we know more about the case, I think the better discussion is this:
Putting aside for a moment that the woman was cheating, does entering into marriage carry some implicit surrender of all your personal privacy? Is every communication - whether written, spoken or otherwise, subject to examination and scrutiny? Now, I fully anticipate the argument that if you're not doing anything wrong, what do you have to hide. Fair enough. But what of personal communications between friends? Do we have to share EVERYTHING...every utterance, every key stroke, every thought with our spouses??
I say no. I say we do not surrender our personal privacy when we enter into marriage. I say we remain individually "sovereign" and share what we desire to share and how we choose to engage in that information sharing and how important or how much is required by the relationship will reach it's needed equilibrium or, it will not and obviously the union would fail.