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have a copy of one email from her to him discussing her own marital problems

This is how affairs start; shared confidences. It goes from there.

If you tell him, he might think you are trying to get even for his backstabbing and he might not believe you. "I don't give a damn if you believe me or not," is a good start to any such conversation. "The reason I am telling you this is because from my recent experience with adultery, I would tell my worst enemy if I thought his wife were cheating or thinking about cheating. It just goes with how I feel about adultery."

If he gets drunk and slugs her, I have mixed emotions. The dominant one is that she should have bailed a long time ago.

Your mileage may vary - these are just suggestions.

Larry