What is incumbent upon a relative whose car overturned in a collision with another car, and he does not know the fate of the other driver?

If your relative caused another person's car to overturn, he is liable for the blood money (diyah) of anyone who died or for the injured body parts, and compensation for the damage and depreciation of the car. He must repent for leaving the scene of the accident without knowing the fate of the car's driver or compensating him. He must strive to identify the injured party and compensate them, and pay whatever is necessary of blood money or expiation. He should know that the rights of created beings must be fulfilled or sought forgiveness for in this world before they become a burden in the Hereafter, as the Prophet, peace and blessings be upon him, said: "Whoever has wronged his brother concerning his honor or anything else, let him seek forgiveness from him today before there will be no Dinar and no Dirham (i.e., on the Day of Resurrection). If he has good deeds, some of them will be taken from him for his brother; and if he has no good deeds, some of the bad deeds of his brother will be taken and added to his burden."