Toriel's heart clenches in her chest. He is most certainly not fine, not with an injury like that. He's quite intelligent enough to avoid burns in normal circumstances, so something's driven him to this in the middle of the night.

"Was there something in the fire, Sans?"

It would be best to get him talking, she thinks...anything to get her a chance to look at that hand if he'll drop his guard. She probably shouldn't be too forward with him.