Toriel says to the human feeling stressed over the situation. She says to them.
"What a miserable creature, torturing such a poor, innocent, youth..." She smiles at Frisk.
"Do not be afraid, my child. It is I, TORIEL, your friend and guardian."
Frisk feels determined, there is no fear, they are a brave child, full of relief of seeing Toriel again. Toriel sighs silently and continues.
"At first, I thought I would let you make your journey alone..."
Her expression changes to a stressful one for a moment. Toriel says to you.
"But I could not stop worrying about you. Your adventure must have been treacherous." Toriel frowns and she says in an unhappy voice. "...and ultimately, it would burden you with a horrible choice. To leave this place, you would have to take the life of another person. You would have to
defeat ASGORE."
Toriel smiles again while talking.
"However...I realized...I cannot allow that. It is not right to sacrifice someone simply to let someone leave this place."