Why does your Protestant friend presume that by celebrating Mardi Gras a Catholic necessarily sins? The day before Ash Wednesday has often been observed in a festive way in many countries, including the Southern United States, in honor of the coming penitential season. That is because the Catholic worldview takes seriously the admonition of the author of Ecclesiastes:
Partying is not necessarily sinful; it is what one does at parties that may or may not be sinful. In answer to your friend’s larger presumption that Catholics sin, then confess, then sin some more, click here. You might also gently point out that a lack of sacramental confession in Protestant churches does not appear to hinder Protestants from sinning and then seeking to become right again with God. It is a fact that in this fallen world that people sin; and that the Christian will seek to repair the damage from that sin. The Catholic Christian will do so by sacramental confession, which he knows that Jesus Christ instituted for the forgiveness of sin.