Is seeking the help of a raqi (healer using Quranic recitation) who claims there is sorcery behind a couple's separation permissible or forbidden, and is requesting the return of the husband considered a sin if sorcery is proven?

There is no harm in seeking assistance from legitimate Ruqyah practitioners (exorcists) to treat magic for the purpose of getting a divorced wife back, provided that the Raqi (exorcist) is legitimate and not a charlatan, and that the Ruqyah is in accordance with Islamic law. It is permissible to undo magic with the Qur’an, Dhikr (remembrance of God), and speech that is not objectionable. As for what was mentioned about the Raqi knowing unseen matters concerning your ex-husband, it is likely that he is a charlatan, and it is safer to stay away from him.