It would heavily depend on context and the situation. And in addition to the other replies, I think it would differ based on if Frieren was alone or traveling with someone, and who that was.

If she and Fern met them at a resturant, things might work out. But I don’t think there would be much left of them if she met them alone on a remote path. At that point she’d probably attack first and ask questions later. Although she might hesitate to harm him in the process. So yeah, it depends on how they meet and if she or someone else with her can detect the difference.

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).

I think it depends a lot.

If Frieren is transported into the KnY world and fights a bunch of local demons, then she’d likely conclude they’re the same shit as in her own world. Then I bet she’d kill Nezuko first and then ask questions.

However, if it’s Nezuko and Tanjiro being transported into the SnF world, I think Nezuko might stand a chance. Frieren would likely notice Nezuko isn’t the same sort of demon as she typically fights, get a bit more on the defensive side, and in the meantime Tanjiro can intervene and explain stuff.