A - Thanks for the question. Here is the passage you are referring to, in context, with emphasis added:
""You are the salt of the earth. But if salt loses its taste, with what can it be seasoned? It is no longer good for anything but to be thrown out and trampled underfoot. You are the light of the world. A city set on a mountain cannot be hidden. Nor do they light a lamp and then put it under a bushel basket; it is set on a lampstand, where it gives light to all in the house. Just so, your light must shine before others, that they may see your good deeds and glorify your heavenly Father. "Do not think that I have come to abolish the law or the prophets. I have come not to abolish but to fulfill. Amen, I say to you, until heaven and earth pass away, not the smallest letter or the smallest part of a letter will pass from the law, until all things have taken place. Therefore, whoever breaks one of the least of these commandments and teaches others to do so will be called least in the kingdom of heaven. But whoever obeys and teaches these commandments will be called greatest in the kingdom of heaven. I tell you, unless your righteousness surpasses that of the scribes and Pharisees, you will not enter into the kingdom of heaven."The passage is at the beginning of the Sermon on the Mount, which is the longest uninterrupted preaching of Christ before the crowds. It covers a wide range of subjects, including the Beatitudes and the Commandments.
-Matthew 5: 13-20
This particular passage is emphasizing the call of every Christian to holiness and obedience to God's laws, which are written on our hearts. We must follow these directions of God if we are to be an example to the world.
Then in verses 17-19 Jesus emphasizes the continual value of the teachings in the Old Testament writings and prophets. The Old Testament is inspired, just as the New Testament is, by the Holy Spirit and is has the same authority behind it - God Himself.
The Rabbis who came before Jesus taught that the there were lesser and greater laws within the Old Testament writings. Thus, Jesus is using this Rabbinical teaching and going further. He is saying that not a single part of the law can be ignored. The Old Testament lays down general principles for most of the laws - for instance "keep the Sabbath Holy". But, the scribes (Jewish lawyers) had many oral traditions that spelled out exactly how these principles were to be kept in practice. A few hundred years after the time of Christ, these oral laws were written down in what is called the Mishnah. Then later commentaries on the Mishnahs were called the Talmud, which are thousands of pages long.
But, even before they were written down, the oral traditions had the force of law in Jewish culture during the time of Jesus. He is saying that these oral traditions focus too much on the externals and the heart is what matter, thus we are told our righteousness needs to exceed that of the Scribes and Pharisees.
The Catechism comments on this in Paragraphs 577-582, which I recommend you read. Here is one paragraph:
581 The Jewish people and their spiritual leaders viewed Jesus as a rabbi. He often argued within the framework of rabbinical interpretation of the Law. Yet Jesus could not help but offend the teachers of the Law, for he was not content to propose his interpretation alongside theirs but taught the people "as one who had authority, and not as their scribes". In Jesus, the same Word of God that had resounded on Mount Sinai to give the written Law to Moses, made itself heard anew on the Mount of the Beatitudes. Jesus did not abolish the Law but fulfilled it by giving its ultimate interpretation in a divine way: "You have heard that it was said to the men of old. . . But I say to you. . ." With this same divine authority, he disavowed certain human traditions of the Pharisees that were "making void the word of God".Now, as to the least and the greatest. There are "many rooms" in heaven as John 14:2 says. These both are referring to the fact that even though all in Heaven will see God face-to-face (the beatific visions), those who lived more virtuous and holy lives, will be more perfectly united to Him.
Thus, in the Catechism it says:
954 All of us, however, in varying degrees and in different ways share in the same charity towards God and our neighbors, and we all sing the one hymn of glory to our God. All, indeed, who are of Christ and who have his Spirit form one Church and in Christ cleave together.So, your friend who is interested in Mormonism might want to read some of the Catholic responses to the beliefs of the Mormons.
Catholic Answers has some good resources, including:
*The Mormon God
*Distinctive Beliefs of the Mormon Church.
I hope this helps.