I think most people here will say to not tell your wgf you're doing it, as she will likely try to talk you out of it or advise her AP ahead of time.
I was actually friends with the OBS, but I still didn't feel comfortable having that conversation on the phone (and we also have a 7 hour time difference now). I sent him an email and FB message. I said, "You may already know this, but if you don't, I'm sorry to be the one to tell you. X and X have been having an affair since X, and I found out recently that they have still been communicating, even though I thought it was over on X. Please feel free to reach out to me if you want (and I provided my phone number) by phone or email.
In your case, you could also add the information about the phone/email her husband was using to give credence to your message. In my case, he had suspected something but never had enough proof. He was very grateful for the information. Good luck...it's a really hard thing to do. If the affair is still going on, the exposure will likely put an end to it once and for all.