Yes, it is true that the Virgin Mary remained a virgin her entire life. The first person to deny this teaching was a heretic named Helvidius. The great St. Jerome - who Protestants accept as one of the greatest Biblical Scholars of the early Church - refuted all of the arguments put for by Helvidius to deny the perpetual virginity of Mary (which happen to be the exact same arguments used by the Protestants today).
The following is a link to the writing of St. Jerom against Helvidius. After reading it, you ought to pass it along to your friend.
Just to help bolster confidence in the preserved truth of this doctrine, this is also a dogmatic teaching of the Eastern Orthodox Church, as well as a belief in the Oriental Orthodox (Non-Chaledonian) communion, and in the Assyrian Church of the East (Nestorians). All teach and proclaim that the Mother of God is Ever-Virgin. So it’s not just a Roman Catholic peculiarity. All of the ancient apostolic churches believed it.
The Douay-Rheims footnote for Matthew 1:25 by Bishop Challoner in 1752 pretty much clears this problem up, and with indirect explanation using Scripture:
"Until she brought forth her first born son: "From these words, Helvidius and other heretics most impiously inferred that the blessed virgin Mary had other children besides Christ. St. Jerome shows, by diverse examples, that this expression of the Evangelist was a manner of speaking among the Hebrews: to denote by the word ‘until’ only what is done, without any regard to the future.
Thus it is said (Genesis 8:6-7) that Noah sent forth a raven, which went forth and did not return until the waters dried up on the Earth; that is, it did not return. Also, (Isaiah 46:4) God says “I am until you grow old” - who dares infer that God should then cease to be (once you are old)? Also, in the First Book of Machabees (5:54), ‘And they went up to mount Sion with joy and gladness and offered holocausts, because not one of them was slain until they had returned in peace.’ That is: not one of them was slain either before or after they returned.
God says to His divine Son: ‘Sit on my right hand until I make thy enemies thy footstool’ Shall He no longer sit at the right hand of God after his enemies are subdued? Yes, and for all eternity.
St. Jerome also proves by Scripture example that an only begotten son was also called firstborn, or ‘first begotten’; because, according to the law, the first born male was to be consecrated to God. ‘Sanctify unto me, saith the LORD, every firstborn that openeth the womb among the children of Israel.’ "
Also where it says in the Bible that Jesus had “brothers”; in the original language, the word “brothers” referred also to cousins and any relatives… sometimes Protestants bring up that point in trying to prove that Jesus had siblings, but when you look at the original - not the translation - it does not prove this at all.
you can check newadvent.org and look up st Joesph he died at 111 years old and before Jesus started his ministry which would make him around 90 when he married Mary. I don’t think she had any other children.
you can also check st Anne Marys mother and Joachim her father. Ann was around 80 when she conceived Mary trough the holy spirit.
Even if Joseph married Eve when he was 17 with raging hormones, how can anyone seriously think that he would mess with God’s wife? There is a reason why Mary is referred to as the Most Chaste Spouse of the Holy Spirit.