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This is a difficult situation. I agree with you that I think his wife should know, especially before bringing children into the picture. However, it would seem like you are betraying him if you "went behind his back", especially because it could drastically affect his marriage. I think the best situation would be for him to come to the conclusion himself that he must tell her. I'd advise you to talk with him openly about your concern, and ask him to converse with his wife. If he refuses, you may be able to get others who "know" to also encourage him - almost like an intervention. If you approach him with his best interests in mind, he may not become defensive.
Also, you may try to discuss this with his therapist or psychiatrist. They may have a relationship which will get better results.
In summary, however, I do think his wife needs to know this information before raising children with him, but this should be done with his approval, and hopefully by him.