I ran across a Protestant person who said Augustine was closer to Protestantism than Catholicism. Below is my response. Hopefully this additional data will help your friend.
Unity with the whole Church in faith and sacraments was a fundamental principle of his thought and action. He used the expression “orbis terrarum”–-the whole world—for this in several works. His famous line securus judicat orbis terrarum (the whole world judges securely) occurs in his first major work against the Donatists: “The whole world judges securely that they are not good who separate themselves from the whole world.” (Contra Parmeniani, 3, 4, 24) The faith of the whole church that Augustine assented to and taught was sacramental, hierarchical, and transmitted God’s word through scripture and tradition in the church.
Augustine stressed the necessity and efficacy of baptism against the Donatists (De baptismo), he teaches the real presence of Christ in the Eucharist (Enarrationes in psalmos 99, 8; Serm. 227; 272; De Trinitate 3, 4, 10; In Ioannis 11, 3); the sacrificial nature of the Eucharist (Questionum in heptateuchum 3, 57; Contra Faustus 20, 21; De Civitas Dei 10, 5 & 20; Letter 98:9), and that Holy Orders is a sacrament (Contra Parmeniani 2, 13, 28; De bono conjugali 24, 32).
But most of all, a central principle in Augustine’s theology and exegetical method is the role of tradition. There is of course his well-known statement: “If you should find someone who does not yet believe in the Gospel, what should you answer him when he says: ‘I do not believe?’ Indeed, I would not believe in the Gospel myself if the authority of the Catholic Church did not influence me to do so.” (Contra ep. mani 5, 6) He argued vigorously for infant baptism convinced that its origin was apostolic because it was handed on by tradition (De baptismo 2, 7, 12; 4, 24, 32; De Genesi ad litteram, 10, 23, 39). This apostolic tradition is guarded and handed on by the succession of bishops of Rome beginning with Peter. In one instance Augustine asks rhetorically what keeps him in the church and answers first the consent of the church throughout the world, the church’s authority, and then “The succession of priests keeps me, beginning from the very seat of the Apostle Peter, to whom the Lord, after His resurrection, gave it in charge to feed His sheep, down to the present episcopate.” (Contra epistulam manichaei, 4, 5). He specifies even further in his letter to Generosus (Letter, 53, 1, 2): “For if the lineal succession of bishops is to be taken into account, with how much more certainty and benefit to the Church do we reckon back till we reach Peter himself, to whom, as bearing in a figure the whole Church, the Lord said: “Upon this rock will I build my Church, and the gates of hell shall not prevail against it!” The successor of Peter was Linus, and his successors in unbroken continuity were these:—Clement, . . . [etc., etc., all the way down to the current Pope at the time of his writing ] Anastasius. In this order of succession no Donatist bishop is found.” (See also Letter 54, 1, 1; Contra Iulianum, 2, 10, 33-34; In Ioannis, 37, 6)
Augustine, is famous for stressing the role of grace, but not at the expense of freedom. He begins a work on grace and freedom with these words: “With reference to those persons who so preach and defend man’s free will, as boldly to deny, and endeavor to do away with, the grace of God which calls us to Him, and delivers us from our evil deserts, and by which we obtain the good deserts which lead to everlasting life: we have already said a good deal in discussion, and committed it to writing, so far as the Lord has vouchsafed to enable us. But since there are some persons who so defend God’s grace as to deny man’s free will, or who suppose that free will is denied when grace is defended, I have determined to write.” (On Grace and Free Will, 1) He also writes: “A man can come to Church unwillingly, can approach the altar unwillingly, partake of the sacrament unwillingly: but he cannot believe unless he is willing credere non potest nisi volens].” (In Ioannis, 26, 2) “He who created you without your cooperation does not justify you without your cooperation. He who created you without your knowing it, he does not justify you without your wanting it.” (Sermon 11, 13)
It is hard to imagine a theological and exegetical method more diametrically opposed to that of the Reformation than Augustine’s. I would say that someone who assented to and taught the faith of the whole church (orbis terrarum) that was sacramental, hierarchical, and transmitted God’s word through scripture and tradition in the church guarded by the succession of bishops of Rome, and whose diocese was not in schism from Rome would qualify as Roman Catholic.