It is difficult to say whether or not he can trust her now, but from what you have said, he seems intelligent enough. If she is not to be trusted he will figure it out pretty quickly. Maybe voice your concerns about it with him, but let him know you have faith in his ability to determine whether or not he can trust her. Better that than appearing like you are trying to interfer in what they have going on. In my humble opinion.
There can be no failure to a man who has not lost his courage, his character, his self respect or his self-confidence. He is still a King - Orison Swett Marden