What is the expiation for the fasting of someone who masturbated during the daytime in Ramadan while she was an ignorant child?

First, what her father's friend did was a grave sin and a major transgression. She bears no sin because she was not yet accountable.

Second, masturbation is forbidden and invalidates fasting according to the majority of scholars if it results in the emission of semen. However, if she was unaware of this, some scholars (including Ibn Taymiyyah) hold the view that it does not invalidate the fast, and no compensatory fast is required.

Third, she must seek much forgiveness, perform acts of obedience, abandon prohibitions, and adhere to uprightness.

Fourth, marriage proceeds according to Allah's decree. One must be content and submit, while also taking the necessary means, such as supplication and obedience.