After thinking about it a bit, I think the young girl was his next or current victim. One of the things he said about "those people" is how they try to stay away but they can't so they put themselves in situations where they're tempted. Dating a woman with a child in that age bracket would definitely fall in that category. He may not have actually raped her yet, but he is in denial of the fact that he can't stop himself. That was my interpretation, but I think it's purposefully left a bit vague. I also do not think he told the woman about Una. Again, I think he is trying to convince himself he is a good person so by saying he had, he is trying as much to convince himself as her. Like she was saying her therapist told her, adults lie and they don't even know they're lying (or something like that).
I was so happy I got a slip for the playbill by Sophie Anne Caruso. I thought for sure I recognized her from Lazarus so it was good to get the confirmation. Wasn't her character "the girl" in Lazarus too? :)