How can one respond to someone who sought divine guidance (istikhara) between the Ibadi school of thought and the school of Ahl al-Sunnah wa al-Jama'ah, and whose heart found tranquility in Ibadism, citing as evidence of an Ibadi not being an innovator of disbelief (bid'ah kufr) the good end of an Ibadi individual and seeing them in a dream in Paradise? And what books do you recommend for responding to this argument?

The Ibāḍiyyah are one of the misguided sects, and dreams cannot be relied upon to distinguish truth from falsehood. Knowledge of the truth is achieved by referring to the Book of Allah and the Sunnah of His Prophet, peace and blessings be upon him. Istikharah (seeking guidance) is not prescribed for obligatory or recommended matters, as they do not admit indecision. Indeed, deeds are judged by their conclusions, and repentance may be a cause for a good ending. To understand the methodology of Ahl al-Sunnah wal-Jamāʿah, please refer to the mentioned Sunni creed books.