Toph had been keeping an eye and yet simultaneously ignoring the prisoner in the lower cells.
That had not been Azula's intended destination; while she was being moved, Zuko had the 'ingenious' solution that he could remove the expectations–and thus the insanity–from his sister by forcibly humbling her.
The private, lavish cell that had been prepared for the Fire Princess lay empty as she was shunned to the dank, disgusting, unused maximum security cells.
Late at night, when she was alone in the office, Toph would sometimes just stop and think about the poor prisoner.
Someone had to.