Edna’s reaction to Robert’s impending departure to Mexico is angry astonishment.
Look in chapter 15 (it might be labeled “Chapter XV” in your copy of the book), and find the second paragraph, the one that begins with “As she seated herself . . ."
There, we’ll see that Edna reacts with shock and disbelief, as if it couldn’t possibly be true that Robert would plan this trip and leave so soon, all without informing her:
She laid her spoon down and looked about her bewildered. He had been with her, reading to her all the morning, and had never even mentioned such a place as Mexico.
As you can see, Edna literally stops eating because she is so taken aback by the news. In literature, when characters are eating together, the scene often suggests communion, or a state of everyone getting along. But when someone suddenly stops eating or refuses to eat with others, like Edna is doing here, it’s often a signal that the communion has been disrupted: that the person who won’t eat is deeply upset. In this case, Edna is deeply upset with Robert. She’s hurt that he’s planning to leave her, and she's hurt that the news of his departure is coming from the others, instead of from Robert himself.
Even though it would be prudent for Edna to hide her shock, since her relationship with Robert is supposed to be secret, she makes no effort to do this:
Edna's face was a blank picture of bewilderment, which she never thought of disguising.
With this information, we’re getting a good hard look at Edna’s character here, at her foolishness and the brazen way in which she conducts her affair with Robert.
You can tell that Edna is deeply upset because she won’t even speak to Robert directly:
“When is he going?” she asked of everybody in general, as if Robert were not there to answer for himself.
Here, we glimpse more of Edna’s characterization: she’s childish, refusing to speak to people who have upset her.
Again, she makes no effort to hide her astonishment from the group as she decries Robert’s plans:
“Impossible!” she exclaimed. “How can a person start off from Grand Isle to Mexico at a moment's notice, as if he were going over to Klein's or to the wharf or down to the beach?”
Silently, in her mind, Edna criticizes Robert and his explanation of when he made up his mind to leave:
“At four o'clock this afternoon, Monsieur Farival,” Robert replied, in a high voice and with a lofty air, which reminded Edna of some gentleman on the stage.
As you can see, she’s starting to accept the reality of Robert’s impending departure, but she’s angrily casting Robert as some phony on stage, some actor, as if to reject the idea that the real Robert could possibly hurt her in this way.
You can tell that she is settling down a bit, accepting the news with a tad more...
dignity, when she makes the effort to keep eating:
She had forced herself to eat most of her soup, and now she was picking the flaky bits of a court bouillon with her fork.
Notice again how the author uses food to signal communion in the group: Edna finally decides to eat, reluctantly and passively participating in the convivial nature of the group at the table. And notice again how Chopin characterizes Edna as childish and petulant: she picks at her food with a fork, playing with it food instead of eating it like a mature adult, as if to express her defiance and bad mood to everyone else at the table.
As the group shares lively stories amongst themselves about Mexico (Robert’s destination), Edna refuses to participate, as if she is too preoccupied and worried, and again she behaves childishly. She blames the others for being foolish, when it is she herself who is being childish by not participating in the conversation:
Edna wondered if they had all gone mad, to be talking and clamoring at that rate. She herself could think of nothing to say about Mexico or the Mexicans.
Finally, Edna manages to speak to Robert directly, but she speaks tersely, almost frostily, to reveal how upset she is with him, not even saying goodbye or wishing him a safe journey:
“At what time do you leave?” she asked Robert.
“At ten,” he told her. “Beaudelet wants to wait for the moon.”
“Are you all ready to go?”
“Quite ready. I shall only take a hand-bag, and shall pack my trunk in the city.”
He turned to answer some question put to him by his mother, and Edna, having finished her black coffee, left the table.
She went directly to her room.
As you can see, Edna promptly leaves the group and closes herself in her stuffy room, like a child who stomps away from the family dinner table after being told she can’t have her way. Her behavior makes it painfully obvious that she is having an affair with Robert, and it's reckless and self-indulgent for her to behave that way. But that's just how she is.