@Ade, your friend’s assumptions, factual or not, is not the issue. Your friend wants explained to them how what appears to be an unjust God, just. He is using this scenario so you must deal with it… to avoid the question he will just use another name and ask the same question.
First: God shows no partiality. (2 Chronicles 19:7) so what would appear unjust in our eyes. If Hitler did indeed repent then yes, he would be in Heaven. And his luxury on earth, Hitler, himself would see has merely shadows and dust from Heaven.
Now if Hitler did repent and God still abolished him to Hell…then God would be unjust. So accepting Hitler into His Kingdom would be the most just thing God could do.
Romans 7:24-25 answers not if God is just or not but Who and How is someone saved that anyone would deemed be wretched or sinful. Romans chapters 7 and 8 would be a great little read to get the fuller scope of the sinner/saved and God relationship.
Your friend is also looking at deeds contributing to ones status to God. Which is also a wrong attribute of God’s character, for God sees deeds as filthy rags.
If God would have came down and stopped Hitler by some sort of divine intervention then God would be unjust. God is sovereign. We have free will which is limited to human ability but is God’s will which is limitless. If God only took pity on the man that was thrown into the gas chamber and welcomed him into Heaven then God also would be unjust. Same thing if He would allow victims of mass shootings or bombings or car accidents into His presence based upon which ever unfortunate circumstances they may have been in…if they would die in their sin… they get death. For the wages of sin is death. This goes back to Romans 7 too. And how now does God view a person who has truly repented regardless of there viscous deeds on Earth.
To your friend, if God can truly forgive Hitler that same God can also forgive you. (Now understand he might not want to “share space” with someone with likes of Hitler so be cautious to say this)
Overall your friend mind is thinking one way (injustice in the world) and his heart is feeling another (justice in the world) this is what you need to bridge and get to the bottom of his question because it goes a lot deeper then Hitler and gas chambers. Teach him the character of God. In doing that, you will be able to give him a proper answer…regardless if he accepts your answer or accepts God or not. Your friend is an atheist too, so even the existence of this “unjust” god he had made up in his mind is not even the same God of the Bible…he doesn’t even believe in. So you will have to establish some grounds first and stand on the Bible and let your speach be True and with grace and respect.