On PG. 91 what did fella do that was both heroic and sad?

I'm pretty sure that you are referring to When Fella crossed over from the "good" side to the "bad side". Vladek is recalling the time all Jews were to report to the stadium to get their passports stamped. If the Nazis deemed you useful to work you were put on the good side. If not you got the "bad stamp" and went to the bad side. Fella climbed over a fence to get to the bad side because he didn't want to leave his daughter and her kids alone. People or rather mice/people from the bedside never returned.