I used to work for a firm that performed computer forensics. This by no means makes me a legal expert, but here's some perspective for anyone considering this kind of work in general:
You should consult a lawyer to determine whether the PI laws in your state apply in the situations you'd like to perform work in. The last thing you want is for something you did to end up in court and then have it turn out that you needed a PI license. Operating as a PI without a license could have legal ramifications. Some states require a PI license for digital forensics work.
Also, consider whether you'd want to testify at all before getting involved. You might think of it as a simple job, but for the jaded spouse this could mean
divorce. If the evidence you gathered (or helped to gather) becomes leverage in divorce proceedings, you may be asked to produce a report, an affidavit, or even testimony. Actual testimony is not so common, but the possibility does exist and it's a roll of the dice. Even a report about what you did and what you found can be scarier than you think; if enough is at stake, the other side can hire an expert who
can and will tear apart every minor mistake and omission.
My law professors from grad school would certainly find this "is it legal" discussion interesting. I personally wouldn't touch it, because it is definitely illegal to wiretap anyone, even a spouse. Additionally, if you install the popular Spector for marital spying, you're helping your client to
breach the license agreement (and you're breaching it yourself).
You shall inform anyone who you may record using the Software that their Internet and computer activity is subject to being recorded and archived.
You shall install this software ONLY on a computer that you own or on a computer which you have been given explicit permission to install. You shall NOT install this software on any computer you do not own or on any computer you have not been given permission to install.
We would take in a good number of cases that were clearly for marital disputes, which usually consisted of deleted data off of cell phones (side note: iPhone, Android, and even BlackBerry - they'll all give up the goods, but iPhone is a songbird). Consult a lawyer before jumping into this business. You want to make sure that your agreement
protects you, particularly if your client outright stole the device from their significant other.