A great deal would depend on what that someone had to say first. The most important question is, is this person repentant. To such a one I would encourage them to remember that Jesus suffered the wrath of God that was due to us for our sins, that those who rest in the finished work of Christ are not only forgiven, but that their sins have been removed from them as far as the east is from the west, that they are not only forgiven but immutably and infinitely loved of their Father, who because of Christ loves them as much as He loves His Son. I would encourage them to not be afraid to enter into the sadness of their loss, but, having repented and been forgiven, not to return to their shame. I know the pain of losing a child and would want only to enter into the sorrow of others who have also lost a child.
If, however, the person has not yet repented of their sin, I would call upon them to repent and believe the gospel. I would tell them that they already know their guilt, that they have not just sinned but have committed what may be the most reprehensible sin one could imagine. They have not only murdered one who bears the image of God, but have murdered such a one who was utterly vulnerable and defenseless. They have not only murdered an image bear that was utterly vulnerable and defenseless, but have done so to their own kin, their own child. They, who were designed to nurture and care for their child, instead destroyed her. I would remind them that among the many things the living God hates is the hands that shed innocent blood (Proverbs 6:17). I would remind them that the all-knowing God finds this so shocking that He says of child sacrifice that it never entered His mind (Jeremiah 19:5). I would remind them that what they think they have done is secret will be revealed on the last day when the child, their own conscience and the living God will convict them.
The first answer, I pray you will note, in no way diminishes what was done. It does not downplay the sin, but rather magnifies the grace of God in Christ. The second answer, on the other hand, is no mean-spirited rhetorical torture. It isn’t piling on but is, like Peter’s potent sermon at Pentecost, careful, accurate, and loving. It is an appeal to whatever tattered remains of conscience this person may have left, in the hope that perhaps God might grant the gift of conviction, give a new heart and lead the person into the second conversation. It is designed specifically to push back against the very lies of the devil that encouraged the murder in the first place.
The sad truth is that too many believers cannot handle this simple truth. Calling sin sin is a sin to the contemporary church, which simply leaves the lost lost. The gospel is our only hope, and is given only to the hopeless. Jesus didn’t come to die for our mistakes, our bad decisions, our lapses in judgment. He came to die for our sins.