Sasha is not involved in the impending war situation.
He is a Hufflepuff! That is not his job! Ed Bones is for some reason trying to make friends with Narcissa Malfoy's protege, but Ed Bones is insane. Sasha is over here making no noise and playing with broken watches and generally pretending not to exist, thank you very much.
Unfortunately for him he is extremely observant and completely unable to turn that off, so he nevertheless finds himself, one day, sitting in the courtyard where the hubbub really ought to be sufficient not to hear anything other than the person sitting next to you, listening to a conversation.