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.

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).