Hope your friend likes some good patristics!
Miscellaneous [Unknown] Apostolic Constitutions (Book VI)
“It is recorded that Paul was beheaded in Rome itself, and Peter, likewise, was crucified, during the reign [of the Emperor Nero]. The account is confirmed by the names of Peter and Paul over the cemeteries there, which remain to the present time. And it is confirmed also by a stalwart man of the Church, Gaius by name, who lived in the time of Zephyrinus, bishop of Rome. This Gaius, in a written disputation with Proclus, the leader of the sect of Cataphrygians, says this of the places in which the remains of the aforementioned apostles were deposited: ‘I can point out the trophies of the apostles. For if you are willing to go to the Vatican or to the Ostian Way, you will find the trophies of those who founded this Church’” (Disputation with Proclus [A.D. 198] in Eusebius, Church History 2:25:5).
Jerome, St [347-420 AD] To Pammachius Against John of Jerusalem
“I follow no leader but Christ and join in communion with none but your blessedness [Pope Damasus I], that is, with the chair of Peter. I know that this is the rock on which the Church has been built. Whoever eats the Lamb outside this house is profane. Anyone who is not in the ark of Noah will perish when the flood prevails” (Letters 15:2 [A.D. 396]).
Jerome, St [347-420 AD] The Dialogue Against the Luciferians
“[Pope] Stephen . . . was the blessed Peter’s twenty-second successor in the See of Rome” (Against the Luciferians 23 [A.D. 383]).
Jerome, St [347-420 AD] Against Vigilantius
Does the bishop of Rome do wrong when he offers sacrifices to the Lord over the venerable bones of the dead men Peter and Paul…
Jerome, St [347-420 AD] De Viris Illustribus (Illustrious Men)
“Simon Peter, the son of John, from the village of Bethsaida in the province of Galilee, brother of Andrew the apostle, and himself chief of the apostles, after having been bishop of the church of Antioch and having preached to the Dispersion . . . pushed on to Rome in the second year of Claudius to overthrow Simon Magus, and held the sacerdotal chair there for twenty-five years until the last, that is the fourteenth, year of Nero. At his hands he received the crown of martyrdom being nailed to the cross with his head towards the ground and his feet raised on high, asserting that he was unworthy to be crucified in the same manner as his Lord” (Lives of Illustrious Men 1 [A.D. 396]).
“Clement, of whom the apostle Paul writing to the Philippians says ‘With Clement and others of my fellow-workers whose names are written in the book of life,’ the fourth bishop of Rome after Peter, if indeed the second was Linus and the third Anacletus, although most of the Latins think that Clement was second after the apostle” (Lives of Illustrious Men 15 [A.D. 396]).
Jerome, St [347-420 AD] Letter 15
“Since the East, shattered as it is by the long-standing feuds, subsisting between its peoples, is bit by bit tearing into shreds the seamless vest of the Lord . . . I think it my duty to consult the chair of Peter, and to turn to a church [Rome] whose faith has been praised by Paul [Rom. 1:8]. I appeal for spiritual food to the church whence I have received the garb of Christ. . . . Evil children have squandered their patrimony; you alone keep your heritage intact” (Letters 15:1 [A.D. 396]).
… that is with the chair of Peter. For this, I know, is the rock on which the church is built!
Jerome, St [347-420 AD] Letter 16
…“He who clings to the chair of Peter is accepted by me.” …
…He who clings to the chair of Peter is accepted by me…
Jerome, St [347-420 AD] Letter 97
…the whole East admires and praises Rome may gladly take to her heart. And may the chair of the apostle Peter by its preaching …
Jerome, St [347-420 AD] Against the Pelagians (Book I)
“‘But,’ you [Jovinian] will say, ‘it was on Peter that the Church was founded’ [Matt. 16:18]. Well . . . one among the twelve is chosen to be their head in order to remove any occasion for division” (Against Jovinian 1:26 [A.D. 393]).
Augustine of Hippo, St [354-430 AD] Against the Epistle of Manichaeus Called Fundamental
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.
Augustine of Hippo, St [354-430 AD] Letter 53 (A.D. 400)
"If the very order of episcopal succession is to be considered, how much more surely, truly, and safely do we number them from Peter himself, to whom, as to one representing the whole Church, the Lord said, ‘Upon this rock I will build my Church’ . . . [Matt. 16:18]. Peter was succeeded by Linus, Linus by Clement, Clement by Anacletus, Anacletus by Evaristus . . . " (Letters 53:1:2 [A.D. 412]).
Augustine of Hippo, St [354-430 AD] Answer to Petilian the Donatist (Book II)
“If all men throughout the world were such as you most vainly accuse them of having been, what has the chair of the Roman church done to you, in which Peter sat, and in which Anastasius sits today?” (Against the Letters of Petilani 2:118 [A.D. 402]).
Augustine of Hippo, St [354-430 AD] Tractate 7 (John 1:34-51)
“Who is ignorant that the first of the apostles is the most blessed Peter?” (Commentary on John 56:1 [A.D. 416]).
Augustine of Hippo, St [354-430 AD] Tractate 9 (John 2:1-2)
“Among these [apostles] Peter alone almost everywhere deserved to represent the whole Church. Because of that representation of the Church, which only he bore, he deserved to hear ‘I will give to you the keys of the kingdom of heaven’” (Sermons 295:2 [A.D. 411]).