Well, there are many Saints out there to whom to pray for such an intention.
The Little Flower, whom the Roman Pontiff proclaimed as the celestial Patroness of Missions, would be a good Saint. Consult the following thread:
http://www.cathinfo.com/catholic.php/The-Little-FlowerOf course, you know how efficacious the Miraculous Infant of Prague is, so why not have recourse to His divine intervention.
Also, making a novena in honor of the person's Angel-Guardian would be also good.
The best way to make a Novena is to say the Holy Rosary throughout nine consecutive days, together with acts of mortification and charity. The Holy Rosary, as Sister Lucia said, has been given great power in our days, perhaps thanks to the Consecration of the human race to the Immaculate Heart of Mary that Pope Pius XII made in virtue of his supreme authority over the round orb of the earth as the Vicar of Our Lord. I believe this consecration is still in effect and shall always be, just like the Consecration Pope Leo XIII made of the human race to the Sacred Heart. That perhaps enabled the world to receive the graces necessary to become less unworthy of the visit of Our Lady of Rosary at Fatima, and her promulgation of the grand epitome of great simplicity and practicality of Divine Revelation as found in the Holy Scriptures, particularly the Gospels and the Epistles, and in sacred Tradition as taught by Holy Mother Church.
So, I would go for a Rosary Novena in honor of Our Lady, Queen of Angels, that she might command the Angel-Guardian of the person in question to illumine his/her mind and open it to the light of faith.
Just my two cents!