Frisk just shrugs. "Not for real. Just for jokes." Literally all they've ever heard out of Sans's trombone is wah wah wahhhhhhhh. They've never even really seen him walking around with it; he just appears, plays it, and ducks away again. It doesn't strike them as particularly weird. They're used to him doing things like that. Papyrus says that he plays pranks across space and time.

"No. That wouldn't happen." The kid seems entirely confident in saying that. If Sans died, they'd just reload to a time before that and fix it. They know they could do it. Their last save isn't even in the Underground anymore. It wouldn't take very long.

They're not leaving anybody else behind.

Besides, dying wouldn't make Sans turn into a human in an imaginary country. They don't know what happens to monsters after they die, but probably not that. "Look." They take a few steps back, holding their hands up in front of their chest as if cupping the air. Obviously they don't really want to fight him, but some of the monsters they met didn't really want to fight either. They just didn't always get that they could hurt humans like that. So Frisk knows the trick to it now, even if there's no fighting anymore, and it's only a few seconds before the red glow of their soul becomes visible.

"This is what human souls look like. Monster ones are different." Reincarnation where your soul changes that much isn't even reincarnation. He wouldn't be Sans.