Is it permissible for a Muslim to seek forgiveness for another person, and is it valid to dedicate the reward of Tasbih (glorification), Tahlil (proclamation of God's oneness), and Qur'an recitation to a living person, knowing that this person is a girl?

It is permissible for a Muslim to include other Muslims in the reward of every act of worship he performs, whether it is voluntary or an obligatory act that allows for deputization, such as Hajj, or one that does not allow for deputization, such as prayer, supplication, seeking forgiveness, charity, and reciting the Quran. It is preferable for the reciter to recite the Quran and remember Allah seeking reward for himself, then supplicate for the mentioned person for guidance and success, because dedicating the reward of these acts of worship is permissible, even if it was not explicitly reported from the Salaf.