Wife wrote the password down
This story's been covered extensively - including previous coverage here in the Reg.
The wife wrote her password down, and Walker found it.
He had good reason to check her email, besides the infidelity. The ex-husband she's been carrying on with has a record of child abuse. Walker notified her *first* husband, her child's father, that she was again putting his son in danger from this criminal.
The real story here is that Walker is being tried by a prosecutor - Jessica Cooper - who's a vile media slut looking to build a career. She, and Clara Walker, are the real villains here. If I lived in Oakland county I'd be canvassing for anyone who was willing to run against Cooper in the next election. Of course, this is a systemic problem with the US's adversarial justice system, and its reward structure for state attorneys.
(For the record, I don't care one way or another about Clara Walker's infidelity - that's between her and her husband. I do care about her bringing the man who hit her son back into the picture. Yes, it can be very hard for people to escape an abusive situation; but once you're out, stay the hell out, idiot.)