What is the ruling on dealing with a girl who speaks of unseen matters such as magic and its location, mentions events that only she knew about, and cites the phrase, "There is no god but Allah, and none knows the unseen except Allah"?

Magic is real, and its cure through extraction is beneficial. The jinn speaking through the afflicted person is also real and a fact. However, do not accuse a Muslim of practicing magic based on the jinn's words through the afflicted person, because the jinn is not truthful. Attributing magic to a Muslim requires an admission or clear evidence. Extracting magic is a good thing, and continuing ruqyah (exorcism) with the Quran and Sunnah is very beneficial.