Mended (Transitive verb):  to put into good shape or working order again; to improve or strengthen (as a relationship) by negotiation or conciliation.

From The Shawl: “What had thrust in her heart? Had something broken inside her, too, as it had in him? Even then, he knew that this broken place inside him would not be mended, except by terrible means…. page 3

Now I understand that his father’s heart could not be fixed, to have it working again like any normal happy person, that his heart won’t improve.