Asalamualaikum wrt wb,
All praise be to Allah.
This man has committed a terrible and heinous crime, and he must repent to God, and the only way for him to do this is by learning about Islam, and becoming Muslim. The best thing you can do for him is educate him about Islam, and tell him about the eternal accountability in the afterlife. Whatever punishment he has in this world is insignificant compared to the great loss in the afterlife if he dies in disbelief.
I know of an Imam who teaches Muslim converts in prisons, and he tells them: Your going to jail is the best thing that happened to you, for it was the means of your learning about Islam and accepting the truth. If a person accepts Islam, all his previous sins will be forgiven.
What is important after Tawbah to Allah is Istiqaama, staying upright and firm on the truth. I ask Allah Most High to forgive all of our sins, and make us firm on the acts of worship and reading of the Quran, and prayer, that we practiced in Ramadan, and immerse us in His Infinite Mercy that encompasses all of His Creation.
And Allah knows best.