We do not believe that salvation is, in a strict sense, merited; it is in fact a free gift.
This is demonstrated by the fact that a baby who has just be baptized is immediately destined for Heaven; if the child died before reaching the age of reason, it would go straight to Heaven.
While salvation is a free gift, it doesn't mean that we are forced to accept it or need not do anything to accept it. A person who receives a wrapped present will not gain from it if they don't unwrap and open it. Opening it does not earn the gift but it is a necessary part of receiving it. For us, Baptism (and Confession) are ways that God has established to unwrap or receive His gift of salvation. (They are not the only means, but they are the means God has primarily revealed to us.)
We can also reject the gift after we receive it by what we do, just as someone can return a gift given or choose not to use it. We do this by deliberately and knowingly choosing to sin gravely: what is known as committing a "mortal sin". This is manifest from what St. Paul writes the Corinthians in 1 Corinthians 6:8-11, where he corrects his Christian audience who are
"cheat[ing] and do[ing] wrong",
and warns them that
"wrongdoers will not inherit the kingdom of God? Do not be deceived: Neither the sexually immoral nor idolaters nor adulterers nor men who have sex with men nor thieves nor the greedy nor drunkards nor slanderers nor swindlers will inherit the kingdom of God." 1 Corinthians 6:8-11
So therefore, there are sins that we can do as Christians that will nullify our salvation if we commit them.
"You have heard that it was said to the people long ago, 'Do not murder, and anyone who murders will be subject to judgment.' But I tell you that anyone who is angry with his brother will be subject to judgment. Again, anyone who says to his brother, 'Raca,' is answerable to the Sanhedrin. But anyone who says, 'You fool!' will be in danger of the fire of Hell." Matthew 5:21-22
"If your right eye causes you to sin, gouge it out and throw it away. It is better for you to lose one part of your body than for your whole body to be thrown into Hell. And if your right hand causes you to sin, cut it off and throw it away. It is better for you to lose one part of your body than for your whole body to go into Hell."
"We know that we have come to know him if we obey his commands. The man who says, 'I know him,' but does not do what he commands is a liar, and the truth is not in him. But if anyone obeys His word, God's love is truly made complete in him. This is how we know we are in him: Whoever claims to live in him must walk as Jesus did." 1 John 2:3-6
"If they have escaped the corruption of the world by knowing our Lord and Savior Jesus Christ and are again entangled in it and overcome, they are worse off at the end than they were at the beginning." 2 Peter 2:20
"For if we sin willfully after we have received the knowledge of the truth, there no longer remains a sacrifice for sins, but a certain fearful expectation of judgment, and fiery indignation which will devour the adversaries. Anyone who has rejected Moses’ law dies without mercy on the testimony of two or three witnesses. Of how much worse punishment, do you suppose, will he be thought worthy who has trampled the Son of God underfoot, counted the blood of the covenant by which he was sanctified a common thing, and insulted the Spirit of grace? For we know Him who said, 'Vengeance is Mine, I will repay,' says the Lord. And again, 'The LORD will judge His people.' It is a fearful thing to fall into the hands of the living God."
Several verses also indicate that we must persevere to the end to be saved (emphasis mine):
"Blessed is the one who perseveres under trial because, having stood the test, that person will receive the crown of life that the Lord has promised to those who love him." (James 1:12)
"Do you not know that in a race all the runners run, but only one gets the prize? Run in such a way as to get the prize. Everyone who competes in the games goes into strict training. They do it to get a crown that will not last; but we do it to get a crown that will last forever. Therefore I do not run like a man running aimlessly; I do not fight like a man beating the air. No, I beat my body and make it my slave so that after I have preached to others, I myself will not be disqualified for the prize."
(1 Corinthians 9:24-27)
"Be diligent in these matters; give yourself wholly to them, so that everyone may see your progress. Watch your life and doctrine closely. Persevere in them, because if you do, you will save both yourself and your hearers." (1 Timothy 4:15ff)
"All men will hate you because of me, but he who stands firm to the end will be saved." (Matthew 10:22)
"Even though we speak like this, dear friends, we are confident of better things in your case -- things that accompany salvation. God is not unjust; he will not forget your work and the love you have shown him as you have helped his people and continue to help them. We want each of you to show this same diligence to the very end, in order to make your hope sure. We do not want you to become lazy, but to imitate those who through faith and patience inherit what has been promised."
"For you have need of endurance, so that you may do the will of God and receive what is promised. " (Hebrews 10:36)
"See to it, brothers, that none of you has a sinful, unbelieving heart that turns away from the living God. But encourage one another daily, as long as it is called Today, so that none of you may be hardened by sin's deceitfulness. We have come to share in Christ if we hold firmly till the end the confidence we had at first." (Hebrews 3:12)
"But my righteous one will live by faith. And if he shrinks back, I will not be pleased with him. But we are not of those who shrink back and are destroyed, but of those who believe and are saved." (Hebrews 10:38)
"Do not be arrogant, but be afraid. For if God did not spare the natural branches, he will not spare you either. Consider therefore the kindness and sternness of God: sternness to those who fell, but kindness to you, provided that you continue in his kindness. Otherwise, you also will be cut off." (Romans 11:21)
Anyway, that should be enough verses for that topic for now, but I have more!
To address his comment about all we need is to have faith and believe, the problem with this statement is that it is taking the Scriptures out of context. There are verses that suggest this but you can't isolate them without contradicting other Scriptures. Scripture has to be taken as a whole.
For example, the requirements for salvation in Acts 2:38 list Repentance and Baptism as requirements. Both of these go beyond believing and having faith. Romans 6:4 and Colossians 2:12 indicate that it is through Baptism that we participate in the Death and Resurrection of Jesus Christ. Mark 16:16 lists Baptism as a requirement for salvation, as does 1 Peter 3:21. The verses I have already quoted indicate that the one who is baptized must also persevere in his faith and his deeds to remain saved. If he falls, then there is Sacrament of Reconciliation (John 20:19-23) to restore him to communion with the Church.
I encourage you and your friend to read the Catechism of the Catholic Church, or at least the relevant sections, such as 1987-2011 and 1257-1270.