To me, this part of the story is really quite sad. It feels like it had potential to end well, but it really has not ended that way at all.
When Isabel gets the letter, she and her friends act very nastily, as seems normal for them. She decides to make fun of the letter by reading it to them. They make all kinds of snide comments.
But then Isabel seems to come to an epiphany up in her room. She realizes how bad she is being. At that point, you think maybe things will come out well.
However, she runs off to go swimming instead of writing back, and you sense the chance to revive their marriage is gone.