Do you know their address? Send her a certified letter that only she can sign for. Return receipt requested so you know if she got it or not. This is what I did because the OMW in my case lives 1000 miles away and only has a cell phone that I couldn't find the # for.
Thanks for the suggestion. Unfortunately, I do not have the GF's address. I'm limited to do sleuthing with what's at my disposal (email, text, phone records) but since my wife hasn't contacted her I have no way of knowing. I haven't resorted to hiring a PI. I'm even glad that my wife admitted her affair before I could fork out money to hire a PI.
Just as a clarification, the GF does not live with the OM. The GF has a SO (not married) but is seeing the OM on the side. (It's all convoluted. These adulterers are living messed up lives.)
