I agree with what cat and grace have said. He knew her before, he has lied to you so he is no doubt lying to her.
That is part of what happened to me. He needed to move out to be on "his own" and yet, had a room mate and slowly let her move in as well.
How did I not flip out him? Well, I did flip out him because it made me feel better. Too bad for him.
Anyhow, after my flipping out, I totally ignored him. I no longer emailed him, called him nor responded to his emails and calls in a timely fashion as I used to. I got out and did things for me. Joined a wine club, went running, went for drives and long walks. Met with friends for dinners and coffee. Visited out of town friends, read books to help me, read books for fun, etc.
It wasn't long after that and after the fact that he knew that I started seeing someone and moving on that he got back in touch.
