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Wow… huge legal issues. You should consult with an attorney in your state who is familiar with privacy laws. I see the situation from a law enforcement stand and I would not touch that situation )
If you state requires private investigators to be licensed, I would also check with them, but make sure they are licensed and have knowledge of the private sectors legal issues. You will also have to check local and state laws as it applies to accessing a computer network. In Ohio what you are suggesting would get you arrested. However, laws as they apply to marriage varies state to state and if she has access to the machine, she might be able to give permission to access it freely.
Sorry I didn't really help much, but be really careful and do your homework for your state.
Found this later down the line…. might help
http//www.forensicfocus.com/index.php?name=Forums&file=viewtopic&t=140
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#1 and #4 are going to get you in trouble eventually.
Those actions could be construed as wiretapping as you are intercepting and in a sense recording a conversation. Quite different than analysis of stored data.