Paul prayed this prayer.
Eph 3:16 That he would grant you, according to the riches of his glory, to be strengthened with might by his Spirit in the inner man;
Eph 3:17 That Christ may dwell in your hearts by faith; that ye, being rooted and grounded in love,
Eph 3:18 May be able to comprehend with all saints what is the breadth, and length, and depth, and height;
Eph 3:19 And to know the love of Christ, which passeth knowledge, that ye might be filled with all the fulness of God.
Eph 3:20 Now unto him that is able to do exceeding abundantly above all that we ask or think, according to the power that worketh in us,
Eph 3:21 Unto him be glory in the church by Christ Jesus throughout all ages, world without end. Amen.
The rooted and grounded in love part is very important.
I would counsel him to not give up. There are actually two words in Hebrew for repentance. Maybe more , but at least two main ones that I am aware of. One is teshuvah which was mentioned already. Which means to turn or return actually. It means to return to God's right way of doing things. Clearly there is a definite point in time when someone begins to turn to God. But it is also a process and not just a one time event.
There is another Hebrew word , nacham. It means to sorrow.
2Co 7:10 For godly sorrow worketh repentance to salvation not to be repented of: but the sorrow of the world worketh death.
We are all capable of human sorrow. Godly sorrow , though is imparted by The Holy Spirit. This person needs to pray that God will give him godly sorrow for his sin and not merely human sorrow. To ask and receive this is a matter of faith. We can't work our way up to it no matter how long we try. It is given by God.
Even Teshuvah is a walk of faith and not of striving. Who can actually walk in his paths of righteousness except that the Holy Spirit empowers and guides ?
In these situations like the one you described , we pray like the man in the Bible who prayed " Lord , I believe , help thou my unbelief ." In other words , I want to believe , help me Lord.
I would counsel him to keep his heart humble and stay in a place of faith. The scripture says that God is a rewarder of those who diligently seek him and also that God does not turn away a broken and contrite heart.
But to answer your main question. The stakes are very high. Not everyone makes it to heaven. We all need to work out our own salvation with fear and trembling. But don't miss the point that I made about it requiring faith. Without faith , we cannot even repent properly. Because human repentance is not enough. We need to ask God to change our hearts and give us a new heart. One of the keys is coming to the end of our self effort and asking God to work repentance in us. There is a big difference between a work of faith and a work of the flesh.
Rom 1:17 For therein is the righteousness of God revealed from faith to faith: as it is written, The just shall live by faith.
Hab 2:4 Behold, his soul is puffed up, it is not upright in him; but the righteous shall live by his faith.
Heb 10:38 Now the just shall live by faith: but if any man draw back, my soul shall have no pleasure in him.
Heb 10:39 But we are not of them who draw back unto perdition; but of them that believe to the saving of the soul.
Gal 3:11 But that no man is justified by the law in the sight of God, it is evident: for, The just shall live by faith.
The preaching of repentance only reveals the problem. Once we discover that we cannot even repent properly on our own , then we realize that we need Him to empower us to repent properly. That is the place of humility. What Andrew Murray wrote about in his book absolute surrender.
Those who believe that their human sorrow is enough are deceived. God must work in us a godly repentance. That is why Jesus said that he came for the sick and not for the righteous. Those who know that they are sinful and cannot even repent properly learn to despise their own hard heart and ask God for a new one. A soft heart that responds to God.
We all fight this battle. Some are just more honest with themselves and others about it. The difference comes with how we respond to finding out that we are hopeless to change ourselves. One path has us die to our self and be raised in Christ. It then becomes no longer I that liveth but Christ liveth in me. The other path seeks to justify self and the flesh.