If your story is told only from Character B's point of view (3rd person limited, aka 3PL), you don't have to show how Character A tricked them.
That's one of the great things about 3PL, your character can be tricked, lied to or manipulated without you showing that. The reader is your character, and shouldn't know what that character doesn't know.
If your narrator is omniscient, and knows what everybody is thinking all the time, then to play fair with the reader, you have to tell them when a character is lying.
To me, that ruins the story, I won't write an omniscient narrator.
In 3PL, your reader should be privy to everything your POV character knows and feels.
But in Star Wars, our POV character is Luke Skywalker. In the movie, when Luke accuses Darth Vader of killing his father, and Vader says "No. I am your father," the audience is just as blind-sided as Luke is. And that is as it should be.
(Some will argue that was foreshadowed, but the scenes they claim are foreshadowing are so ambiguous I disagree. Like the battle in the cave, seeing his face behind Vader's mask; to me is showing the danger of turning to the Dark Side, not at all foreshadowing a genetic link.)
It is okay to blindside your POV character and your reader that identifies with them, as long as the blindsiding actually makes sense. We already know Luke is an orphan, and knows little about his father, mostly only what Obi Wan and Yoda has told him. They have been vague. This reveal makes sense. It did not have to be foreshadowed, or shown, to work.