Not even the intercession of the Virgin Mary could redeem the irredeemably prideful Devil, but that takes nothing away from the favor God has granted our Blessed Mother as Mediatrix of His infinite graces.
From the writings of St. Alphonsus de’ Liguori, whose feast we celebrate on August 2:
The blessed Virgin herself revealed to St. Bridget, that no sinner in the world is so great an enemy to God, that if he has recourse to her and invokes her aid, does not return to God and is not restored to His favor. And the same St. Bridget heard one day Jesus Christ saying to His mother, that she could obtain the Divine favor even for Lucifer, if he would humble himself so far as to ask her help. That proud spirit would never stoop to implore the protection of Mary, but if such a thing could happen, Mary would take pity upon him, and the power of her prayers would obtain from God His pardon and salvation. But what cannot happen to the devil may well happen to sinners who seek the help of this mother of mercy.
— The Glories of Mary