I would ask them if two or more Christians have a differing interpretation of scripture (tradition) how do we know which tradition is the correct one?
If either tradition was in the Bible, it would be there, but it is not there, so pointing it out in the BIble is not going to help. Remember, that an interpretation of scripture (tradition) is not the same as the scriptures being interpreted. The Bible does not come with a list of “100% acceptable to God” interpretations of scripture in it.
The tradition of “sola scriptura” actually makes null a boatload of Bible verses, most notably Matthew 18.
If there is a disagreement, first confront the believer one on one. Then get a couple of people to confront him alltogether. If he still doesn’t listen: TAKE IT TO THE CHURCH.
With 30,000+ denominations, WHICH CHURCH?
Here’s a fictional dialog between two Christians having an issue. One wants to baptize his dying little infant sister so she could go to heaven." Imagine them going according to Matthew 18.
“We should not baptize babies”
“Yes we should! I want my sister to be baptised.”
…
…
“Let’s call some of my brethren…”
…
…
“OK, now let’s take this dispute to the Church.”
“OK, I’ll take it to the baptist church, they say infant baptism is unbiblical”
“But I’ll take it to the Anglican church who says it is!”
“Oh yeah? I’ll take it to the nondenom church who denies infant baptism”
“OH yeah? How about the Lutheran Church which says it is OK?”
…
Ohoh, which Church got it right? Who is holding to the apostolic interpretation of scripture for this dispute?
That’s why tradition and magisterium are needed. The Catholic Church got it right, and the gates of hell have not prevailed.