Your friend is expounding a verse by St. Paul somewhere in one of the letters to the Corinthians where it says that spiritual things must be explained with spiritual things.
Actually this is not entirely a bad principle, when it applies. In the Vat II document Dei Verbum, the Fathers of the Council asserted that, that you have to appeal to all of scripture to understand any of it – it hangs together – because it has the One God as its author.
There’s maybe a verse, then the local context of that verse, then all of the Bible. But, it’s not that that principle is bad, it just isn’t enough.
Scripture itself does not give us that much information, to explain every other verse. Look at the account of Jesus on the road to Emmaus, walking with two disciples and explaining all the places in the Old Testament which refer to Jesus and his mission.
Where are all these verses that Jesus was referring to? We don’t have such a list, we have to take the best guess at which passages Jesus was referring to.
The underlying issue here is the importance of Tradition as a source of divine inspiration, alongside scripture. The ultlimate interpretation of spiritual things is given by the Church. Jesus said he would send the Spirit to teach them all the things he couldn’t do, and that the Spirit would give the Church the fullness of truth. It all doesn’t come out of a book, even the Bible.
The apostles were preaching and spreading the gospel for decades before all the book sof the New Testament were completed. As Msgr. George Agius said in his book Tradition and the Church, the early Church didn’t even need the Bible to spread the gospel, because of the tradition – handing on from one person to the next – of the good news.
Agius has a lot of illustrations about how important tradition is. One of them is to resolve disputes between two people who are arguing about scripture – it is the Church that decides how to interpret the scripture and settle the dispute. This happened with issues like the gnostic problem and the Arian heresy of the fourth century.
Agius wrote this book way back in the 1920’s, and he is really animated and expressive. In one place he even says that the Bible is a dead book without Tradition to explain it. and, it’s not any tradition, like Martin Luther’s tradition, but it’s Apostolic Tradition which the Catholic Church adheres to.
Scripture itself says to adhere to traditions that have been passed on. From the concordance in the Ignatius Study Bible, these verses are 2 Thess 2:15 and 2 Thess 3:6.
In several minutes of searching, it seems that your friend’s argument is based on 1 Co 2:13-15. Yeah, sure, but that’s not all that Paul teaches, as I’ve said, e.g. the citations from Thess.
DON’T ARGUE. Don’t say anything unless you have really prepared. Too bad you probably don’t have the Agius book handy.
from Dei Verbum section 12
But, since Holy Scripture must be read and interpreted in the sacred spirit in which it was written, (9) no less serious attention must be given to the content and unity of the whole of Scripture if the meaning of the sacred texts is to be correctly worked out.
section 9
Consequently it is not from Sacred Scripture alone that the Church draws her certainty about everything which has been revealed. Therefore both sacred tradition and Sacred Scripture are to be accepted and venerated with the same sense of loyalty and reverence.(6)
etc.
here’s the link to the entire document
ewtn.com/library/COUNCILS/v2revel.htm