Is it obligatory to conceal a past mistake if it is exposed, and is that considered a lie, and does repentance prevent one from paying the price for the mistake?

The sister should conceal herself and not inform her fiancé or anyone else about what happened, based on the hadith: "Avoid these abominations... whoever commits any of them, let him conceal himself with the concealment of Allah and repent to Allah the Most High." She must cut off contact with the young man, repent to Allah, and increase in good deeds. It is permissible for her to use tawriyah (ambiguity) or even lie if there is a need and to prevent a greater harm should her fiancé become aware of it. For repentance erases what came before it, and the one who repents from a sin is like one who has no sin.