How do I expose the affair

[mb28]

I know one convo that my friend had with him about a month ago that is almost word for word what she said. She asked him what was up with this OW. He said they were just friends. She told him that it hurt me to talk to her, and that if she was truly just a friend, then it shouldn't be any big deal to end the contact. He agreed and than went on to say he had stopped talking to her that past week. However, the next weekend is when someone sent that letter to the OW's family and now they have been talking non-stop. And that is my H's excuse for talking to her again. He has ruined her life by being her friend and wants to make it better, blah blah blah. I heard this last part from his mom.