One thing to remind your friend is what your relationship would be in the world to come. We know that Salvation is only possible through Christ Jesus
Acts 4:12 Neither is there salvation in any other¹ for there is none other name under heaven given among men, whereby we must be saved.
therefore anyone rejecting Him will not make it to Heaven. For your friend, the argument that Jesus will save everyone after they die except maybe for people like Darwin or Himmler doesn't really apply (even if it's true) (which it isn't) because he already rejected Christ's offer. "No thank you Jesus," he's saying in effect, "I don't want your love; I think it's a rotten deal: I'd rather eat worms!" So we can dismiss that [hypothetical] discussion [with your friend] and get back to you. Of course you're concerned, and I know you're suffering
II Timothy 2:12 If we suffer, we shall also reign with him² if we deny him, he also will deny us
Over whom shall we reign? It will be a joint reign with Christ but without subjects how can anyone reign at all? You will already have explained to your friend that Salvation comes only in Him so all Christians will be there and everyone else will be somewhere else. Clearly if Christians were reigning over other Christians, the second group would not be reigning over anyone, unless there's going to be a 3rd caste of Christians for them to reign over. I'm sure you can where that would lead. However your friend would not even be in the 100-trillion-and-9th caste (necessitating a 100-trillion-and-10th caste) (and he wouldn't be in that either) so the solution is obvious. Christians will ALL be reigning and therefore reigning over The Unsaved. Your friend may wonder how long this will go on?
Revelation 22:5 And there shall be no night there; and they³ need no candle, neither light of the sun; for the Lord God giveth them light: and they shall reign for ever and ever.
Perhaps in your prayers you could ask God to reveal an effective way to communicate this to your friend? It may not be a circumstance he desires. There are all sorts of opinions out there as to who gets to do what in the afterlife and I've referred to a couple here which are heresies (because they go against what God has already revealed) but am not offering an opinion myself. The relevant sources are included with context as footnotes which you may like to read to your friend.
In Christ.
1. Other than Jesus, see verse 10ª
2. Jesus, see verse 10ª
3. The servants of The Lamb⁴ see verse 3ª
4. Jesus is The Lamb see John 1:36
a. or click on hover link in the main text for Scripture in context