Greetings, brother!
You can check out the first 5 chapters of Garrigou-Lagrange’s
Predestination.
And I saw a quote from Augustine in your signature!

He’s the go-to guy on this doctrine (and most any doctrine, really) if you ask me. You’ll find that there is much that you’ll be able to affirm in your friend’s understanding. Reading any of his
anti-Pelagian writings will help. But it may be best to start with
On the Predestination of the Saints and
On the Gift of Perseverance.
“Will any man dare to say that God did not foreknow those to whom He would give to believe, or whom He would give to His Son, that of them He should lose none? And certainly, if He foreknew these things, He as certainly foreknew His own kindnesses, wherewith He condescends to deliver us. This is the predestination of the saints,–nothing else; to wit, the foreknowledge and the preparation of God’s kindnesses, whereby they are most certainly delivered, whoever they are that are delivered” (
On the Gift of Perseverance).I say, send your friend to Augustine! If your friend takes the time to read his writings, what will happen is that he will find much in what Augustine says to be pleasing, which will make him want to read more. But he will at the same time be adjusted and corrected in his own thinking and be exposed to a firm exposition on other Catholic doctrines that he would otherwise not be exposed to.
“[W]ho causes that men should be good save Him who said, ‘And I will visit them to make them good?’ And who said ‘I will put my Spirit within you, and will cause you to walk in my righteousness, and to observe my judgments, and do them?’ Are ye thus not yet awake? Do ye not yet hear, ‘I will cause you to walk’, ‘I will make you to observe’, lastly, ‘I will make you to do’? What! Are you still puffing yourselves up? We indeed walk, it is true; we observe; we do; but He makes us to walk, to observe, to do. This is the grace of God making us good; this is His mercy preventing us” (
Against Two Letters of the Pelagians, Bk. 4).Catholics, he says in this book, “assert God’s grace above free will, as antecedent to all merit, so as truly to afford a gratuitous divine assistance” …] “Let them, then, who disdain, if they do not do any evil and if they do any good, to glory, not in themselves, but in the Lord, learn to be Catholics.”
“We do not say that by the sin of Adam free will perished out of the nature of men; but that it avails for sinning in men subjected to the devil; while it is not of avail for good and pious living, unless the will itself of man should be made free by God’s grace, and assisted to every good movement of action, of speech, of thought… all are born under sin on account of the fault of propagation, and…, therefore, all are under the devil until they are born again in Christ… nor does man at all begin to be changed by the beginning of faith from evil to good, unless the unbought and gratuitous mercy of God effects this in him” (
Against Two Letters of the Pelagians, Bk. 2). “Augustine would seem to have wrested the palm from all. Of a most powerful genius and thoroughly saturated with sacred and profane learning, with the loftiest faith and with equal knowledge, he combated most vigorously all the errors of his age…
How subtly he reasoned on… the will and free choice” (Pope Leo XIII,
Aeterni Patris).
“Hormisdas wrote in answer to Bishop Possessor’s request for direction these weighty words: ‘What the Roman, that is, the Catholic Church follows and maintains touching free will and the grace of God, can be learned from the different works of blessed Augustine’…
“Finally by a Divine impulse, he carried over many years his study of the ruin of the human race after the sin of our first parents, of the relation between the grace of God and free will, and of what goes by the name of predestination. So closely did he study the subject and with such happy results, that he was deemed the Doctor of Grace and was so entitled. He led the way for all other Catholic writers of later ages, to whom he reached a helping and a restraining hand, lest in their discussion of these intricate problems they err one way or the other: either by teaching that free will in man, once his original justice was lost, is but a name and no more, as the early Protestants and the Jansenists held; or that divine grace was not a free gift and was not all-powerful, as the Pelagians kept repeating” (Pope Pius XI,
Ad Salutem Humani).In Christ,
Pete