"They were good after he left."
Translation:Elles ont été sages après qu'il est parti.
There is no special context here. So "they" could be a sport team that "he" was supporting. They were having a bad game. He had to leave at some point. They finished the game very well. "He was disappointed by the game. 15 mins to the end, he had to leave for work and wasn't in the mood for it. But he heard on his way that they got an equaliser right after he left and scored the wnner in injury time". "They were good after he left". "Ils ont été bons après son départ". Bref bons devrait être accepté.