In Dubliners, what was the cause of Gabriel's quarrel with his mother? Who was proven right?

About a third of the way through the story, Gabriel recalls an argument he had with his mother about his marriage to Gretta. As he recalls the argument "a shadow passe(s) over his face." He recalls that his mother didn't think very highly of Gretta. In fact, she thought dismissively of Gretta as "country cute." Gabriel then recalls that "It was Gretta who had nursed (his mother) during her last long illness." This implies that Gretta disproved her mother-in-law's criticism, thus also proving Gabriel to be right when he decided to marry her.
Later in the story, however, we learn that Gretta and Gabriel's marriage is not without its difficulties. Gabriel learns that Gretta has been in love before, with a boy who tragically died. He begins to think that perhaps Gretta has been comparing him all along to this other boy. Perhaps in this sense, the mother might have been right after all, at least in part. She was perhaps right in thinking that Gretta was not the right wife for Gabriel.