Neither of them have an idea what he's saying. Frisk has a way of voicelessly commanding, but that's far from a genuine sign language. He's trying to communicate something, but that's all they understand.

...He looks a little like a skeleton. That means monster, weak. It's enough to get them on their feet and hurrying toward him, sharply shaking their head back at Sparks. No battle here.

Unfair comes next, a snarl that dissolves into half a whine, but they're not going to cave on this one.

"You need to leave. It's dangerous to be up high in a storm."