I know my tagline may be a bit ironic in this case, but it is there for a good reason. I had this bad habit of lying, but I made a promise to God to not lie anymore, so it is there mostly to make me be honest all the way.
I have a reason to defend this woman, and a very simple one, and one that you who live in US should be proud of, everyone has a right to proper defend, no matter what the case is.
I am not saying you are wrong, no, you are all right in the fact that she did a big mistake, but it is not our duty to judge, our duty is to forgive, and I know that it sometimes is impossible, and I know it personally, and no, I have not give full forgiveness in that case, only a “I try to not think about it all the time”. But there is a difference between “mistake” and “sin”. I am sure she knew that what she did was wrong and a sin, however we are not perfect and I know from my own experience how easy it is to sin as she did. It only takes a cold marriage, which in this case in some distance between the OP and the girl, but the same “rule” do apply in both cases. When you hunger for a bit of affection, and someone just close to you, and the feeling that you are wanted are not available it is so easy to seek someone who will give you what you need. That was my reason for cheating, and maybe the same reason explain why she did what she did. A lonely night, booze no doubt, and the need to feel someone close to you. (I did drink sometimes back in the “good old days”.) So that is my other reason to be on her side. I know that it is a very bad excuse, but we do live in a real world with real people around us, not a isolated island. I did love my x-wife, and part of me may still love her, but love is not a one way street, if you give you have all the right to recieve as well. I understand that the OP feel bad and hurt, who would not? Well, actually, all I was afraid of when I was married was that my wife would leave me for someone else, not her cheating on me, that would not have been a “big deal” for me. She, BTW, never did cheat. Maybe our situation was different in many ways. She had one marriage behind her and I had two lenghty relationships in my past. But the basics was there all the time, so you maybe understand why I defend her. Yes, she could have told him before they got married but maybe, just maybe, she was afraid of losing him, and that is why she did not tell him, and at some point she felt she needed to let him know. And as I did write earlier, she saying “it is no big deal” is usually what people say in a effort to diminich the size of the sin/deed. I once did throw a stone at a window, and I ran straight to my grandfather and told him what I had done and I think I did say “but it was only a small window”, can’t be sure though. It is not unusual to try to make it look smaller then it is, a sin I mean. (Do I need to confess that window next time? I never have, and I did not even remember the matter before now, it is about 45 years since it did happen, and I was not even aiming at the window, I did try to hit the boy throwing stones at me.) Back to the issue, she may very well be a good woman, and she can be a good wife and mother, and they do love each other. And there we have reason number three. Nobody on this forum have any power to judge her, what we have though is the words from the Gospels. Did Jesus judge the woman He did meet at Jacob’s well? No, and why did He not. Because nobody else could do it neither. So maybe we could pray for that marriage, as I have done? It is a marriage, that is a thing nobody can deny. It may fall apart, but so can many better marriages do as well. We just don’t know. And because we don’t, we ought to do what Christ did, and are doing all time, forgive. So before you lynch her you maybe would like to think what you would want if you would have done the same thing what she did. What would you want? The same thing we all want, a fair “trial”, proper defence and a second chance. Deep inside you do know that, don’t you. What Jesus did mean when He told us to love our neighbour is what this is all about, how would you like to be treated in the same situation. The OP need our prayers, and his wife need them, and their marriage need them. If your advice make him get that annulment and he later on regret that he was so hard and did not give her the second chance I hope you can sleep well each night because I could not. I must be on the side of her because nobody else is.