The book does not give the proof, and that is where the discussion about it comes from. No scene shows Capitu alone with Escobar. Everything the reader knows about the two comes from Bentinho, who is at once the husband, the narrator and the accuser, and who writes decades later, already convinced that he was deceived.
The reading that she cheated rests on four points. Ezequiel, the couple's son, has gestures and features that recall Escobar, and it was Capitu herself who remarked on the resemblance at the table. Before Escobar's photograph she becomes confused, and Bentinho reads a confession in this. The separation is requested by her own hand. And, in the last chapter, Bentinho states that the girl of Matacavalos and the woman of Glória beach were one, "as the fruit inside the peel."
The reading that she is innocent notes that each of these points admits another explanation. More than fifty chapters before the accusation, Gurgel, Sancha's father, shows Bentinho the portrait of his late wife and says that Capitu resembles her, with no kinship between the two: "in life there are such strange resemblances." Bentinho recalls this when he is deciding and moves on. Capitu asks him to tell everything so that she can defend herself, and he does not. About the photograph, the narrator himself acknowledges that she "with her mouth, however, confessed nothing." And even the eyes that condemn her were first described by José Dias, in the middle of complaints against the Pádua family, and then by Bentinho himself, while in love.
So both readings still stand. What the text allows us to affirm is that Bentinho believes he was betrayed, and that the book shows, step by step, how that conviction was formed: he decides first and rereads the past afterward ("Now I remembered everything that then seemed to me nothing"). If Capitu did betray him, the novel does not say.