How can repentance erase the sin of slandering chaste women, and is this sin among the seven destructive sins?

Your friend is in grave danger due to his abandonment of prayer and slandering of chaste women. Abandoning prayer is disbelief (kufr), as the Prophet, peace and blessings be upon him, said: "Between a man and disbelief is the abandonment of prayer." And Umar said: "There is no share in Islam for one who neglects prayer." Slandering chaste women is among the major sins and destructive acts. His repentance from it is by recanting his accusation in front of those to whom he slandered the chaste women and seeking forgiveness from those he slandered, for Allah Almighty says: "And those who accuse chaste women and then do not produce four witnesses - lash them with eighty lashes and never accept from them any testimony, and those are the defiantly disobedient, Except for those who repent thereafter and reform, for indeed, Allah is Forgiving and Merciful." If he is unable to recant his accusation and seek forgiveness, then it suffices for him to improve his condition, rectify his deeds, feel remorse, seek forgiveness, and resolve not to return to it. Committing major sins does not justify abandoning prayer; rather, prayer may indeed prevent indecency and evil.