What if he doesn’t even believe that we can know whether or not the Bible we have is 100% certified inerrant? What if he says his beliefs are derived from a general consensus among the Christian church (in a broad sense)?
That is probably what a lot of Christians, regardless of label, do believe, among other things.
It’s just that so much of the Catholic apologetic literature on this site and that I’ve seen elsewhere pertains to evangelical and fundamentalist Protestants who hold sola scriptura to be true. If they do not hold the Bible or the Catholic Church as authoritative, how do you convince them that the Church is where they should be? Is there a good starting point?
There is an element of truth in that position. It is that Christ has handed down to His Church the deposit of revelation. . All Christians agree that Christianity is a revealed religion. So our duty is to find out what God revealed. But how can one person determine what is the consensus of all Christians? You need an authority to decide on the contents of tradition. It always comes back to the question of authority - the authority given to Peter and the apostles and to their sucessors.
Since 70% of the Christian church throughout the world is Catholic or Orthodox, that person should have Catholic beliefs (or at least Orthodox).
It would be hard to convince a person to be Catholic if he did not believe in the inerrancy of Scripture because we believe that too. If someone did not believe in the inerrancy of Scripture and the inerrancy of Tradition, he would probably most likely be inclined toward “cafeteria Catholicism” or “cafeteria Protestantism”; IOW, more of a relativist.