Prof's neurological health is declining rapidly. What can I (TA) do?
Let the department head know what you've observed. You can write an anonymous letter if you feel uncomfortable or are worried about burning bridges.

(I've been in this situation, as a student. We - the students in the class - wrote a collective letter to the department head. He was able to arrange extra TA support for the professor for the rest of that semester. He also arranged things so that this professor had other duties in the department going forward, but no more teaching.)