What is the way to calm the fear of the reckoning on the Day of Resurrection, and to atone for money taken unlawfully, which its owner was unable to return? Is it permissible to donate it with the intention of [on behalf of] the owner of the money and to seek forgiveness for him?

If the money was stolen from the girl's father, it must be returned to him by any possible means, and there is no condition to inform him. If you are unable to reach him, then give it away as charity on his behalf. However, if the girl owns this money, for example, if her father gifted it to her and then she gifted it to you, then you are not required to return it.