The sin that leads to death is the sin which CANNOT be forgiven. And this is different from atonement. All sins are atoned for. The legal obligation of death for sin was taken care of for all sin for all time by Christ, once, on Calvary.
But forgiveness is quite different. Forgiveness requires two people -- the offender and the one offended. It does not matter how much forgiveness I may have in my heart for someone who offends me, if that person never comes and asks for forgiveness, I cannot give it-- there is no recipient. My duty is to have the forgiveness ready.
In the converse, if I apologise to someone I have offended and they choose not to forgive me, there is nothing I can do to cause that forgiveness to happen.
In Romans 1 we read that the wrath of God is being poured out on those who suppress the truth. That means two things at the start: first that there is truth these people are seeing and also that they are consciously suppressing it, preferring the lie. Continue on that road long enough and Romans 1 tells us that God will give the person over to the lie and the actions which follow.
Note that Jesus said "I am the Truth...." and that He referred to the Holy Spirit as the Spirit of Truth (read John 14-17). Truth is revealed by the Holy Spirit and through creation (two witnesses...) and what a person does with that is his choice. If a person wants the truth and follows it (whether or not he knows he is seeking God), then the Father will lead him to the Son and Christ will refuse no one who comes. But if the person prefers the lie, then that person is moving away from Truth -- Jesus Christ.
And it is only in Christ that we find forgiveness. So to walk away from Christ, who is the Truth, the Way, and the Life, is to reject the truth, the way, and life itself.
For this person, who has suppressed that truth he was shown and has seen on his own, forgiveness is impossible. For though Christ is ready to forgive, not willing that one should perish (2 Peter 3), and although the sins of the person are atoned for in full, that person is preferring another way, and there is no forgiveness on that route. It does not matter how 'good' he thinks he is or how many excuses he has for his various behaviors. He cannot be forgiven because he has walked away from forgiveness.
How can you pray for a person to be forgiven if the person himself is rejecting, actively and consistently, the source of all forgiveness?
Is that person incapable of repenting? There evidently comes a time when God finally says to the person "thy will be done" since the person refuses to say that to God. We don't know when that point is, although Pharaoh at the time of Moses is a really good example of it. The writer to the Hebrews begs them not to harden their hearts, as their ancestors did in the wilderness.
So yes, there comes a time when repentance appears to have become impossible for a person. But when that time is we dare not presume. We MUST treat everyone as though they could repent and could come to Christ. We are forbidden to judge the heart.
Therefore, in the spirit of loving one's neighbor as oneself, no matter who that neighbor is (God may have put you in your position in life precisely so that you WILL be in contact with this person...), we should pray for them. Sometimes a person's life is darkest just before the dawn. Many of us were in that position. We should always pray.