- a few comments from my side. Good reasons to tell OM's W
From a moral perspective, she has the right to know.
From a consequences perspective, the affair needs to see the light of day.
From a preventative perspective, the affair is far less likely to go on in any way if both your wife and the OM are being monitored by their respective spouses. Less good reason to tell OM's W
The OM's W is an asset to be exploited to get additional information/confirmation (and I really get this, believe me, I do).
You need to bear a few things in mind:
1. You are telling her something that might be devastating to her. A few hours to compose herself apart from her husband is not going to make a huge difference. (If I'd been phoned when my husband was not with me, then the first thing I would have done is call him after receiving that call. Remember that thinking strategically is not likely at that point)
2. Unless she has amazing coping skills, she isn't even going to be able to think about looking for more information for you. She will want to see what you have first. She also needs to have some say in how to proceed from that point on. It is her life too.
3. You are a complete stranger to her. If a stranger asked me to start spying on my husband (even with the information that you're going to tell her), I am far more likely to either confront my husband, or perhaps I may start snooping (but in my own way, at my own pace).
4. She has no inherent loyalty to you. She may share a desire to get to the truth. But, in truth, you do not have her best interests at heart - you have your own. (This is as it should be, by the way).
So, if you're hoping that by telling her, you're getting a spy in the enemy camp, then I think that you are going to be disappointed. Don't try and manipulate her, just call her at home. If you're worried that you might give yourself away if the OM answers, then get someone to play switchboard operator for you and ask to speak to her.