For the sake of this thread, our hypothetical nonbeliever is quite happy with his non belief and he feels pity for his confused mother.
Biblical believing brings clarity. So, if we are talking about a Biblical believer who is about to die and be with Jesus, and knows this, the believer can not be confused. It is written >
"For God is not the author of confusion but of peace," in 1 Corinthians 14:33.
Also, our Apostle Paul has prayed for us who are believers >
"And this I pray, that your love may abound still more and more in knowledge and all discernment." (Philippians 1:9)
So, in God's love we become more and more clear in how we see. So, if you are talking about a true believer, she can not be confused.
So, may be he sees his mother, a true believer, as being confused. This would be fooling himself, which would not be moral; he would be the confused one; and Jesus has compassion for him . . . pit for him; and this is part of why Jesus came to this earth and so suffered and died for us . . . because of His compassion for us.
Among other things, mr. hypothetical is fooling himself, if not knowingly, and this might not be deliberate deception, but it still is wrong - - for him, to fool himself.
Also it can be wrong for others > in case he is going around showing off, even, that he is doing great and is happy, this is helping others to suppose they too don't need Jesus, and so they miss out on how they could have peace with God . . . because of him. And so they will suffer in this life and later, unless they get wise to him being so wrong.
He could be showing off a very inferior way of being happy, when anyone may trust in Christ and be the way God's way of loving makes us actually better than how selfish and self-dependent ones are capable of making themselves happy. So, he would be a bad example, immoral, by being happy in a way so inferior.
And you seem to say you object to people suffering torment in hell. So, if there is hell and he is helping people to suppose they don't need Jesus, that is very immoral because of how he is making it look as though people don't need Jesus who has so suffered and died so we do not go to hell.
So, yes it would be very immoral . . . even if he is in denial so he does not know this.
So, in case he is happy with this, yes this is a major problem, plus extremely conceited if he actually feels he is too good for Jesus who is God's own Son. But Jesus is so superior to and better than us, and yet Jesus left Heaven and came here in the flesh in order to reach us and share His own good with us, and even suffer and die for us, on the cross, so we would not suffer torment after we die. So, Jesus is not at all conceited . . . like humans who can pick and choose who is good enough for us to love.
And if he is happy while being a bad example in other ways . . . this is not helping children and older people to find out how to love.
For just one example > there are people who smoke, and act like they are doing great and they are cool people. But their example could help children to become smokers so later they could suffer and die from cancer, plus go through the financial and emotional havoc of that with those who care about them.
So, if someone is happy with that, and yet
claims to object to people suffering torment . . . that person's bad example of smoking could be
helping people to suffer. Meanwhile . . . on the other hand - - - if the mother has been a believer and a good example, she has not helped people to go to hell; so she would not have a moral reason to keep suffering in Heaven.
And there are people who directly disobey and dishonor how God's word says for us to relate in His love. For one example, God's word says >
"Do all things without complaining and disputing," (Philippians 2:14)
So . . . I see from this > arguing and complaining are anti-love. And so, if parents disobey God's love rule not to complain or argue, and if they do not find out how to love, their bad example can help to keep their children from knowing how to love in a close relationship. And the Bible has various scriptures which can help us how to find out how to relate intimately with another person > for examples > Ephesians 4:31-32, Ephesians 5:21, 1 Peter 5:3; plus any scripture can be used to help us become more real in how God has us love.
In the United States where I am, there is something like a little more than a fifty-percent divorce rate for marriages. And there can be a household culture of independence so spouses and the children spend much time and commitment energy on each one's own interests, instead of doing more sharing and learning how to relate, with one another. And this can feed into how ministers later do their ministering and neglect their own families and time to enjoy God and to refresh with Him > so selfish independence, which can start in the home, is effecting even how relating is in church ministry.
So, such parents into such stuff are a bad example helping kids to not know how to love. So, if your hypothetical believer grew up to become a confused person, possibly she was a product of American independence which isolates spouses from one another and isolates Bible claiming people from God so they are tormented by stress and burnout in their self-dependent pursuits.
In case this is why she is confused, his lying to her will not help her, I would say.
If he truly cares about himself and what she feels, the time of her deathbed is too late . . . especially, if all he has to give her at her deathbed is a lie!!
If he loves her, genuinely, he needs to start before then, so he can help her become a clear person of God's love, so she is not confused at the end of her life.