Your friend, not unlike many today, has a flawed understanding of holiness. In your friend’s case, he apparently thinks that a “good Catholic life” is a little like J. K. Rowling’s description of Hogwarts students earning house points in the Harry Potter series, with points added by the teachers for achievement and deducted for misbehavior. In such an understanding of holiness, reaching heaven is like being handed the House Cup at the end-of-year school celebration.
What your friend needs to understand is that holiness is communion with God and those in union with him. It is first and foremost a relationship with God. Holiness is achieved not by toting up good works like they were brownie points, but by allowing oneself to be drawn into friendship with God. Good works flow from that friendship, and they are important to maintaining the friendship, but we don’t do them in order to earn that friendship.
With all this in mind, the answer to your friend’s question is that going to Mass is a primary factor in a Catholic’s relationship with God. He goes to Mass first and foremost because he loves God. He also goes because he knows that the Church God entrusted to teach in his name requires Catholics to do so on Sundays and holy days of obligation. If this Catholic has spent his life growing in friendship with God, loving God and trying to please him, it is not likely that he will skip Mass without just reason. If he has just reason, he will not have sinned. If he does miss Mass without just reason, and he does indeed love God, he will repent and go to confession at the next available opportunity. If he dies repentant, but is prevented by circumstance from going to confession, his repentance can be sufficient for salvation.
Keep in mind something else. Look at the stories of the saints. How many died “unexpectedly” or, more precisely, “unready”? Even in cases of violent deaths, saints have never been, strictly speaking, “unprepared” to meet their God. And perhaps that is because they always sought to be in constant friendship with him.
