My understanding is that in regards to marriage, one is not allowed to ask the other about their past - marriage is supposed to be a new start, a new life for the couple. The replies in this thread are based on their own personal feelings and not from Quran and sunnah.
If the guy has repented then it is for Allah, and Allah alone to judge him, not you lot. Similarly if the woman in question has repented then it is for Allah to judge her and Allah alone. The very fact that you all are judging him and telling him what to do proves that you lot think best. Marriage is a blessing and if Allah Wills that a man who has committed zina 1000 times is to marry the most pious woman on the planet then so be it.
Thirdly, the woman in this scenario is not innocent. Just because the guy may appear to be more in wrong does not exonerate the woman from sin. It takes two to commit zina and she was a willing participant. It speaks to her character that she is willing to destroy the guy's relationship with his family in order to get what she wants. Sure he should take some responsibility for his actions but so should she! They both need to accept what they've done is wrong and repent to Allah - and that is what they both have (apparently) done. Don't you see that this is the most important thing?
This life is a test, and a temporary one at that. These two have failed a test and have committed zina. Similarly us lot who are looking in from the outside are being tested on how we react to such news. You're all feeling sorry for the wife-to-be, have I missed something? Do you all suddenly possess the power to see the future? This guy may turn out to be the best possibly husband for this woman, who are you to deny that? Allah knows best, not you. Also just because she wears a piece of cloth over her head doesn't make her Miss Religious. And if it turns out that she is pious then she would know about Isthikaar (oh and by the way, what is Isthikaar, ah yes, asking Allah for guidance).
The act of zina is haraam, it is a great sin no doubt. The act of repentance however is a greater good and Allah loves those who repent - this isn't an excuse to sin but rather one who sins then repents is doing so due to fear of Allah. Don't get me wrong, I'm not saying what the guy should or shouldn't do, he should seek someone knowledgable in Islam to answer that, someone who can guide him using examples from the Quran and sunnah, not some personal feelings from angry elitist women (and for some reason, guys) on an internet forum.
My Islamic advice to you - guy - is to perform Isthikaar and to tell your wife-to-be to do the same. Seek a Sheikh or a Scholar in Islam regarding your initial query.