Eh, sounds emotionally manipulative to me. If he really didn't want you to find out, he would have come to the rather obvious solution of ignoring her texts until he was alone. By texting her in front of you and then acting all sneaky-sneaky about it, it's like he deliberately provoked you into being concerned and asking about it. So that he could then explain to you how crazy you are, and keep on doing it, knowing that you would continue to worry about it and start to doubt yourself. Maybe he will have an affair, maybe he won't, but either way he sounds like a douche to me.