Everyone who dies unrepentant from any actual mortal sin will be judged by God and condemned to Hell forever. Everyone else goes to Heaven. For God does not permit innocent children to die without the state of grace. And I contend that any adult who dies in a state of “original sin only” is guilty of the actual mortal sin of omission of never having found sanctifying grace in life, despite ample opportunity. So if you die unrepentant from actual mortal sin, you go to Hell. If you die in a state of grace, you go to Heaven (perhaps by way of Purgatory). There is no third case.
The easiest path to Heaven is to live as a believing and practicing Roman Catholic for your whole life. Such persons are blessed with the fullness of truth in the teachings of Tradition, Scripture, Magisterium, and with the seven Sacraments and the many other benefits of formal membership in the Church.
Refusing or neglecting to pray to God is an objective mortal sin. But if an individual sincerely thinks that God does not exist, or sincerely has a concept of God that precludes prayer (i.e. not a personal God), the lack of prayer in his life is perhaps not culpable to the extent of an actual mortal sin.
How can an atheist or agnostic enter the state of grace? One way is by an implicit baptism of desire. Or he may have been validly baptized with water as an infant or young child. Or he could possibility receive a baptism of blood, if he dies in an attempt to help his neighbor, with true love of neighbor in full cooperation with grace.
How can an atheist avoid grave sin, if he refuses to believe in God or to pray? As long as his refusal is based on his sincere but mistaken conscience, he may still retain the state of grace.
If he loses the state of grace by any actual mortal sin, perhaps a sin not related to religion, how can he return to the state of grace? Just as it is possible to enter the state of grace by an implicit baptism of desire, it is possible to return to the state of grace by an act of implicit perfect contrition.
So it is possible for someone who has never prayed to be saved and have eternal life in Heaven, almost certainly after a long time of suffering in Purgatory.
The idea that you are “damned if you do not pray” is narrow-minded and Pharisaical. Those traditionalists go astray from the mercy of God who claim that conversion to Catholicism, or to Christianity, or to belief in God is absolutely essential for salvation.
Please take a look at this list of my books and booklets, and see if any topic interests you.