Will Allah conceal her (sin) or will she be exposed, after her repentance and regret for a past sin she committed?

As long as you have repented with sincere repentance, then trust in Allah's forgiveness, for "the one who repents from a sin is like one who has no sin." Strive to maintain the obligatory acts and increase in supererogatory acts, for "indeed, good deeds efface bad deeds." Have a good opinion of your Lord, and trust that He will conceal your sins and will not expose you. Live your life normally, and do not worry about the past as long as you have repented to Allah.