Ultimatum is a good idea, but give him a short one. She has to know ASAP, preferably within the month. If she wants to move out, divorce, move back home or stay and sort out the marriage, she needs to do that NOW before the baby is born. After a baby is born is not a good idea. Not only is she going to be overwhelmed with a newborn, almost all women experience some sort of depression right after birth and are quite emotional in the first few weeks or even months. Telling her the news may send her into postpartum depression, and decrease her ability as a mother. Tell her now, it still really is just she that she has to deal with and take care of. Tell her later, you're seriously affecting the baby's life too. Worse case scenario, your ******* of a friend may actually get the baby if the mother is really depressed. No offense, but I would not want to be friends with someone like him, so I don't see why you still care so much about what he would think and do. The fact that he would so blatantly flaunt his invincibility by saying she would never believe you shows that he is not really guilty or think he's a bad person at all. Make copies of the email (on USB, external HD, printout) then tell him that you have the emails and have made copies to ensure its safety, and if he doesn't tell his wife in two weeks you will show her the email and tell her. Don't invite him to your house, talk to him in a public place, like a quiet corner of a cafe or something. He can't harm you there. And honestly, if nothing, do it for the baby. She doesn't deserve to be born to a torn marriage and fighting parents. And the fact that your friend doesn't like his own child because it's a girl is absolutely ridiculous
. He doesn't deserve a child or a wife. They deserve so much more, and for them, you have to tell the truth, or make him do it. If he agrees to talk to her, tell him to give you a date, and tell him that you will talk to her after that date. So that way he can't just be lying to buy more time, and won't give her a twisted version of the affair. Good luck!