Tell them to read their Bible. In 1 Samuel 2 they will read about the two sons of Eli, who were wicked priests. These priests were known to be having intercourse with the women who worked at the Tabernacle (1 Sm 2:22-24). God punished them for their wickedness (1 Sm 2:25, 34, 4:11), but that did not disprove the religion of Israel or that its priesthood was from God. Similarly, the fact that there are bad priests today does not prove that the Catholic religion is false or that its priesthood is not from God.
Bad priests have been around since the Old Testament, and if you had judged the religion of Israel by the quality of some of its priests, you would have missed out on the true religion. Ditto for Catholicism today.