You give them the example of St. Athanasius and tell them to read the story and point them to a book.
The Romans deposed Liberius when they believed he had become a heretic. Liberius wasn't an Arian. But Benedict XVI is a modernist.
It would be a good teaching lesson on infalliblity and its limits.
As Catholics we're bound to accept the magisterium of the Popes. Now how do I explain to a Protestant that I accept the Pope but not his magisterium?
Also you point out to them that the PCED said it is not a sin to assist at their Masses and they fulfill the Sunday obligation.
And yet the Pope says that the SSPX priests have no ministry in the Catholic Church. So they are priests acting outside the Pope's authority, presuming Benedict XVI is Pope.
You also point out to them that there is a difference in the fact that the SSPX is a society with an irregular canonical situation, and individuals in the Society or who assist at their Masses who are Catholics inside the Church.
Sure I'll point out they are Catholics only they reject the magisterium of the Popes while calling them true Popes.
You also point out the difference between Prot "reformers" like Luther who rejected 1500 years of Tradition and Traditionalists who support 2,000 years of Tradition.
Yes, traditionalists support 2000 years of Tradition, but the current Pope signed the joint declaration on justification (which was approved by John Paul II. So was the Pope wrong in the 1500s or is the Pope wrong now? If Luther was wrong then then he's wrong now, if the Pope doesn't say Luther was wrong then he's departed from the Catholic Faith.