Before? He'd healed them the only 'before' they can remember--after their first fight.

It takes several seconds (several more waves of quiet tears) before it clicks. Oh, god. Oh god.

"Ssssfour no. N-n-n-nobody can h-heal dead people. 'M sorry. So so sorry."

They let go of his hand and hug him right. "A-an'--an' I p-p-promised I'be be careful, too..." Sheer disgust chokes them to silence. They promised, and Frisk came to him all bloody and messy and crying anyway.