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Hi there, this sounds very much like a bad conscience to me. And: The fact that he has a girl friend doesn't mean that he doesn't care for his wife anymore. He still can remember the woman she was. Since ou are not the appropriate person to talk intensively about his worries concerning her, he does that with his dinner partner.
She will stay a part of his life, and she will keep his mind busy. Maybe you can tell him that you are, in fact, the right person to discuss his wife because you don't have any reason to be envious. This would be much better than irritating dinner partners with this topic, and help you getting closer.