What should a young man do who has wronged a girl, then she asked him for forgiveness, but he refused?

If you have apologized to the woman you wronged, then there is no sin upon you. Know that acquaintances and conversations between young men and women online or otherwise are a door to temptation and corruption. Therefore, fear Allah and sever your relationship with this woman and other non-mahram women, and beware of the whisperings of Satan.