Best of luck to you, I am sure that will be a frustrating conversation. I will be praying that you are able to maintain your cool. That, I believe, is of the utmost importance when engaging in these sorts of conversations. Debate tends to harden people to their original position, and providing a good example, rather than intellectual argument, is often the best way to win people over to the true Church. You can have the most convincing arguments in the world (and the Church does indeed have all the most convincing arguments in the world!), but if you lose your temper it can just drive the other side deeper into their original convictions.
Now, that said, you obviously can’t let calumnies against the one true Church stand unchallenged, you have to politely, yet firmly, demonstrate their falsity. In such an argument, you have already taken the first step in emphasizing the importance of Holy Tradition. Personally, I have always been confounded by the Protestant fallacy of sola scriptura. How on earth can one understand the scripture without the accompanying tradition? Protestants, after all, have their own tradition, the difference is they don’t believe it is divinely inspired. So a good line of argument might be to question how the opinion of Luther, or Calvin, or Zwigli, or whoever, is any better than anybody else’s opinion. That’s all Protestants have to go on, really, is the Bible and opinion. Christ established the Chruch for precisely this reason, to avoid the sort of anarchy you see in Protestantism.
Be ready to counter the Protestant objection to the deuterocanonical scripture, which they reject. The Christian canon was decided at the council of Nicea, the same council which gave us the Nicene Creed (which the Protestants accept) also gave us the canon. So the reason they reject the deuterocanonical books is based on another of opinion of Luther (and for the more pragmatic reason that the deuterocanonical provide theological books support the Catholic Church).