seems to me there's a simple solution, the hero is fine around demons so just study the hero's excess mana and use that to create forms of plant and insect life that can withstand demon energies and terraform hell back into a lush landscape, with hell restored demons wont need to go to the human world to survive but can trade with humans and both sides can gradually learn to get along (well, I suppose that's assuming there are different materials available in hell vs human world that would be worth trading back and forth for each side, which seems likely to me)