Here are two great questions to put to your Protestant friends:
In John 20:21-23, Jesus gives the Apostles authority to forgive (or retain) sins. Since your Protestant friend likely objects to the Catholic practice of confessing sins to a priest (since, the dichotomy goes, only God forgives sins), tactfully ask her how she interprets Jesus’ words to the Apostles.
If she says something like, “that command and authority was given only to the Apostles and died out with them,” you can respond by asking: “well, if you lived during the time of the Apostles, would you confess your sins to them?”
If she says no, then she is contradicting herself: on the one hand she says Christ gave this authority to the Apostles, but on the other she says she won’t submit herself to them.
If she instead responds that “Christ’s directive was given to all Christians. A Christian can, therefore, confess their sins to any other Christian. Even James 5:16 says as much.” She is now in the difficult position of 1) knowing who, exactly, to confess to and which sins to confess to them, 2) whether she herself should hear others’ confessions, and 3) how she knows that the passage in John and the one in James were applying to all Christians and not just to the Apostles (and their successors) and the presbyters.
As you can see, this whole line of discussion gets right to the heart of the main issue which divides Catholics and Protestants: authority. So it can be quite fruitful to propose these thought experiments to your Protestant friends.
Would You Obey Paul? What About Timothy?
A related question targets sola Scriptura. Protestants believe that the Bible alone is the sole infallible rule of faith. But what if they lived during the Apostolic Age, say in 45 AD. Some of the books of the New Testament may have been written by this time, but certainly not all or even most. Would they follow the teachings of Paul (you could substitute any Apostle’s name here, but Protestants in particular favor (St.) Paul, so he’s a good one to use)?
This sounds simple but actually is perplexing to most Protestants, because they are deeply rooted in the belief that their ultimate authority is the Bible, and not a person. But what if that person were an Apostle, indeed, one who wrote most of the books of the New Testament?
One way they might answer is that, they would follow Paul unless and until he contradicted the Bible. But you can point out that he wrote the Bible and that not all the books were even written yet, so how would they know that what he said contradicted it?
Another response, a bit better, might be that Paul would never contradict the Bible in his teachings because he was an Apostle and God was guiding him, so yes they would obey him because he would not lead them astray (an infallibility of sorts is implied).
A good rejoinder to this answer would be to push the question one degree later: would they follow Timothy, Paul’s disciple? This is even more difficult for them, because Timothy is 1) not an Apostle in the full sense of the title, yet 2) was discipled by an Apostle and 3) has two books of the Bible named after him(!), personal letters from Paul to him. And in them Paul gives many exhortations that indicate Timothy has authority (through the laying on of hands upon him) and that people should not despise him because he is young. So Paul himself believes that Timothy has authority from God.
The problem is that Protestants believe that at some point in the second, third, or fourth century, the Church became corrupted. So they “know” that eventually they would have to say they cannot trust the successors of the Apostles (because they taught error). So while Timothy seems pretty legit to them, what about the many other (unnamed) disciples whom the Apostles chose to lead churches? Perhaps some of them were heretics and could not be trusted.
If they’re smart, they will turn this question back around on you and ask if you would follow Timothy. And here is where you bring up the fact that you would follow the Church. In other words, any given Apostle or successor may indeed commit a sin or cause a scandal or even personally fail in some way (like Peter temporarily refusing to eat with Gentiles even though God had told him it was cool). But the Church herself cannot err in her teachings, because God protects her from error. So the Church can be trusted.
Protestants do not believe the Church can be trusted in this way. So they only have individuals (like Timothy, or Titus, or Clement) whose theology they then have to judge based on their own interpretation of the Scriptures. In other words, “the Church” is just an amalgam of individuals, not a coherent, visible organism that definitively teaches things and binds members to believe them.
As you can see, asking them the question of whether they would follow the Apostles and their immediate successors, or not, is also quite fruitful and will generate a great discussion. It will provoke thoughts in their minds that they may never have considered, and through prayer and God’s grace, will hopefully lead to their entrance into full communion with the Catholic Church.