Is it fair for a young man to repent, get married, and have children, while the girl he was in a relationship with remains unhappy and unable to marry, knowing that he promised her they would continue their relationship as friends?

It is recommended to fear Allah and hasten to repentance and seeking forgiveness. For whoever fears Allah, He will make for him a way out and provide for him from where he does not expect. Furthermore, the relationship with the young man is not permissible, and its prohibition intensifies with persistence in it. Repentance from it is obligatory, and it is not permissible to take revenge on one who repents. It is advised to ask Allah Almighty to rectify the situation and grant a righteous husband, for Allah is near and answers the call of the supplicant.