What is the expiation for one person saying about another, "Indeed, Allah will never guide him"?

A Muslim must beware of presuming to speak for Allah (at-ta'alli 'ala Allah) and despairing of His mercy. For Allah forgives and punishes whom He wills, and His mercy encompasses all things. He is the Forgiving, the Merciful. There is no specific expiation for presuming to speak for Allah, but due to its seriousness, one must hasten to sincere repentance, seek forgiveness, and increase in supererogatory acts of worship, obedience, and good deeds.