The truth always comes out...?

Salam,

This question is for a friend. She is engaged and getting married in just a couple months inshAllah. She is a very good muslimiah alhamdullilah, however she has done a few things in the past. She has gotten intimate with someone before, but it never got to the point of intercourse. She has repented sincerely and is a changed person since then, everybody makes mistakes. I know Allah is all forgiving, and I've heard in a lot of lectures before that nobody needs to know about your past as long as you have asked for repentance.

It has come up with her and her fiance and he is asking if she has done anything in the past. Islamically, does she have to tell him or can she keep it as her own little secret?

Is it true that anything bad you have done will always come up in the future and your secrets will be revealed? Or is that only if you don't repent.