@ossobuco,
Well, if he trusts his wife, then he should have no qualms about the guy staying in the house with her when he is at work. Because if he trusts his wife, then the only way something will happen between her and the ex is if the ex forces himself upon his wife. If he has any reason to believe that might happen, then why would he even consider letting a guy who he believes might rape his wife stay at his house under any circumstance. The fact that he seems to be totally agreeable to the guy staying indicates that it is really his wife that he does not trust. At least that is how I see it.
Regardless of who he does or does not trust, there is nothing wrong with insisting that this guy stay in a hotel, just as all of us here are suggesting.