What is going on if your wife called a male coworker into her office and said she wanted to be friends and asked him to call her and he said not likely?
Your actions are illegal and yes you are reading into this too far. Look for more absolute signs like late arrivals, suspicious activities etc. On a time you know she'd otherwise be free, suggest an outing (movie, dinner, picnic etc.) If there's an unexpected hesitation, there is reason to be concerned. I don't condone spying in a relationship if there hasn't been any suspicious circumstances to give cause. Just because you're in a relationship with someone doesn't give you unlimited access to their email, mail and phone conversations. But realistically, if someone has already had red flags about their spouse's behavior and are basically wanting proof so that they can make a decision about confronting the person, exposing an affair and the future of the marriage, I say go ahead. In this case, your wife is obviously wanting more than a innocent co-worker relationship with this guy--but it sounds like he turned her down. It's up to you to decide if you want to stay in a relationship with a spouse who is making advances towards other men or not. If you decide to confront her, be aware a) You're going to have to let the cat out of the bag about how you know what she said and b) she'll probably get angry and deny everything.