Proof that the traditions we have is from the Apostles?

The question of apostolic origin for Sacred Tradition is foundational. Here are some key sources:

Scripture itself attests to oral tradition: St. Paul writes, "Stand firm and hold to the traditions which you were taught by us, either by word of mouth or by letter" (2 Thessalonians 2:15), and "What you have heard from me before many witnesses entrust to faithful men who will be able to teach others also" (2 Timothy 2:2).

The Catechism explains that "the apostolic preaching, which is expressed in a special way in the inspired books, was to be preserved in a continuous line of succession until the end of time" and that Sacred Tradition "transmits in its entirety the Word of God which has been entrusted to the apostles by Christ the Lord and the Holy Spirit" (CCC §§77–78).

For historical resources, I'd recommend:

• Dei Verbum (Vatican II's Dogmatic Constitution on Divine Revelation), especially §§7–10, which explains how Tradition and Scripture flow from the same divine source
• The early Church Fathers themselves — St. Irenaeus's Against Heresies (c. 180 AD) traces teaching back through bishops to the apostles; St. Vincent of Lérins's Commonitorium (434 AD) articulates the test of authentic tradition
• For a modern treatment, Cardinal John Henry Newman's Essay on the Development of Christian Doctrine addresses how doctrine unfolds while remaining apostolic

Your friend might appreciate seeing how the earliest Christians (within living memory of the apostles) already practiced and defended these traditions.