What does Harper tell Dr. Lenz about his relationship with Erin? Why?

At the FBI safe house, Harper tells Dr. Lenz that he and Erin had a fling before he and Drewe got married. He tells Dr. Lenz that she came to visit him in Chicago, and one thing led to another. They spent most of the next four days in bed, and Erin actually became pregnant as a result. Harper told Dr. Lenz because he had already told Harper that he knew about the relationship previously.