In Frieren’s world, Demons only appear and act human as a survival trait (from what I’ve seen so far). In Demon Slayer, Nezuko is turned into a demon, not born one, and retains much of her humanity in staying with her brother and helping him fight other demons. I think if Frieren were to see that she was turned and not born a demon, they could come to an understanding. Frieren may even make the effort to find a way to revert Nezuko back to being a human.

For context, I’ve only watched the Frieren anime (not started the Manga) and only watched & read Demon Slayer through the Demon Train arc (I got burnt out and haven’t picked it back up yet).