What is the relationship between Frances and Robert in Chapter 5-6 of The Sun Also Rises?

In chapter
5 of the book, Robert talks to his friend Jake (Jacob Barnes) about wanting to travel to South
America but being held back by Frances. When Jake suggests that he should take Frances with him,
Robert says this: She wouldnt like it. That isnt the sort of thing she likes. She likes a lot
of people around. Robert then shifts the conversation to Brett, a woman he seems to have taken a
liking to in chapter 3 of the story. The fact that Robert is interested in Brett points to a
shift in his relationship with Frances. Robert seems to be exploring his dating
options.

This is not the first time that Robert talks to Jake about wanting
to travel to South America. Earlier on, in chapter 2 of the story, Robert presses Jake to
accompany him to South America, but Jake declines the offer, stating that he is quite happy in
Paris. Chapter 2 also talks about a new Robert, come back from America, where his novel has
just been accepted by a good publisher. The new Robert is...