While I agree that Montresor is unreliable and betrays Fortunato, there is one moment that suggests some betrayal of Fortunato towards Montresor:
I broke and reached him a flagon of De Grâve. He emptied it at a breath. His eyes flashed with a fierce light. He laughed and threw the bottle upwards with a gesticulation I did not understand.
I looked at him in surprise. He repeated the movement - a grotesque one.
"You do not comprehend ?" he said.
"Not I," I replied.
"Then you are not of the brotherhood."
"How ?"
"You are not of the masons."
"Yes, yes," I said, "yes, yes."
"You ? Impossible ! A mason ?"
"A mason," I replied.
"A sign," he said.
What happens here is that Fortunato has made a sign associated with the Brotherhood of Masons, an society of men going back many centuries. Fortunato realizes that Montresor has not...
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recognized the sign and thus questions him about it. Montresor says he is a mason, but he is obviously lying. Fortunato's reply, however, reveals some betrayal of his true feelings towards his "friend". He clearly feels superior to Montresor by saying "You? A mason? Impossible?" So why treating Montresor as a friend, he betrays this friendship by assuming he himself (Fortunato) is more superior.
When reading this story, we have to remember that our narrator, Montresor, is not a reliable or credible narrator. He believes that Fortunato has wronged him in some way, but he never tells us exactly what it is that Fortunato has done to him. All we know about what Fortunato has done is:
"The thousand injuries of Fortunato I has borne as best I could; but when he ventured upon insult, I vowed revenge."
Montresor believes that Fortunato has deceived and betrayed, but we can't be sure that it isn't just in Montresor's head either because he is so unreliable as a narrator. Fortunato genuinely seems to think that he is friends with Montresor when he sees him at the carnival. He is caught completely by surprise and even thinks that the burial alive might be a joke, so he clearly has no idea what he's done. It is because of Montresor's beliefin the betrayal and deception that leads to Fortunato's eventual burial alive.