Kanesher - I think your friend's Kallah is smart...I have a friend who was engaged, and his Kallah, coming from a more modern orthodox background, had been involved with guys, on at least some physical level. He knew the basics, and that was enough - he wanted to move on, didn't want to constantly think about it.
But the girl kept mentioning things, including alluding to specific guys that she'd been with who might actually be by the engagement party, wedding, etc (due to family friendships). He tried to explain to her that he really didn't want to have specific images in his mind.
They're no longer engaged (though that's not the only reason, I think).
The less specifics one knows, the better, I think - but obviously, the girl has to be confident that things are over. How to balance that...