How can one respond to a person who sees clear visions and believes in Sufism, knowing that his shaykh tells him things that actually happen and claims connection with saints (awliya'), and is what I say true, that devils help their followers?

The knowledge of the unseen is of two types: absolute, which is exclusive to Allah, and relative, which He may reveal to some of His servants. Righteous visions are divine glad tidings for the righteous, and inspirations (ilhamat) occur to those who adhere to Allah's Sharia, while satanic suggestions (ihayat) are for the allies of Satan. People vary in their dreams: the dreams of prophets are true, most dreams of the righteous are true, while others experience both true dreams and confused dreams (adghath). A disbeliever or a wicked person may see a true vision, but this does not indicate sainthood (wilaya), for sainthood lies in observing obligatory acts (fara'id), supererogatory acts (nawafil), and the stations of faith and piety. True sainthood belongs to the allies of Allah mentioned in the Quran: "those who believed and were constantly conscious of Allah" (10:63). One must beware of spiritual unveilings (mukashafat) and satanic states that contravene the Quran and Sunnah, for charlatans possess supernatural abilities but are disbelievers, and devils descend upon liars. Many Sufi groups today have doctrinal deviations, such as major polytheism (shirk akbar) by invoking others besides Allah and seeking help from the dead.