Yes, you can expect him to be a dick to the person working in his office. Absolutely you can. My H was also worried about appearing rude, but what he really meant was that departing from the script he had developed with these APs or potential APs ("oh, aren't you wonderful"--"No, you're wonderful!") would cause him to lose the feel-good strokes.
He had an affair with his co-worker. Odds are he is still having an affair. If anything he has to do to make you feel safe in the marriage feels embarrassing or uncomfortable, tell him to do it anyway. If he refuses, he's not ready for R. And honestly, hasn't her roommate already told you the A is still going on? Or have I confused this post with another? So to spell it out:
1) Her roommate says the A is still happening and they're planning a life together.
2) The AP is moving closer to you, which at the very least indicates she's not trying to find employment elsewhere.
3) Your H is placing his phone where you can't hear their conversations.
The only evidence you have that the A has ended is your H's word for it and, as my therapist told me, liars lie. Here's what your H can do to prove it:
1) Make her quit. Or fire her if he has the power.
2) Quit his job and find another.
3) Give you complete transparency with full access to his phone and his computer.
4) Become remorseful and not just regretful that he was caught.
Has your H done any of the above? Believe me, I get it--this stuff is almost impossibly hard and moving out of the marriage when your H is swearing he loves you and only you is so tough. It's confusing. It's maddening. But look at the facts and make a decision. Lord how I wish I had someone giving me this advice way back when my H was in his first A.