Frisk doesn't bother to try and defend themself. It doesn't matter that their entire body had been turned to glass, that there were already plenty of volunteers, that without SAVE they were little more than a liability on a mission like that. The facts have already been laid out, and anything else would be just making an excuse. They waited on the ship while Asriel suffered through who knows what sort of horrible things, and pretended like there wasn't anything else they could do. Even an apology would just be a slap in the face.

All they can do is bow their head in shame.