What does Marie do when Meursault tells her about old Salamano and his dog?
Marie, like most of the other characters in The Stranger, acts as a character foil to Meursault. She reacts with sympathy for the dog and Salamano. However, Meursault acts indifferently, thinking that the relationship between Salamano and the dog works well for them and it is not chaotic (in his mind), so there is no need to pass judgment on them.
Marie and the other characters in the novel act as the voice of society and they often are used as the mouthpiece for Camus to comment on how society views non-conformity. They think that because Salamano abuses the dog, then he is a bad person. However, the reality of the matter is that Salamano loves his dog and only abuses it because he wants to know that there is a connection between them, be it an abusive one. We can see that there is genuine love from Salamano for the dog when we learn that the dog has run away. Salamano acts as if does not care for the dog, but Meursault later hears him crying through the wall of the building.
It is this same sentiment that Marie has that does not appeal to Meursault. As he states near the end of the book, we are all destinted to die eventually, so there is no point to ponder aspects about life that does not concern us.