Well, Abq... I agree more with you. You should verify the things you can verify, but you are totally right, if he wants to communicate with her, he will. If your instincts start to twitch, then you'll have to decide how for to go to find out.
I think there is a balance to be struck. He should be willing to be transparent, but at the same time you can't act paranoid. I think the earning of trust is a two way street, and a process. Obviously, the more trust he earns, the more you can trust him. But I think it works a bit the other way as well.
