When we think about plot, the falling action is the section of plot that comes after the climax and before the resolution of the novel. This is normally not a very large section of the story, but it nonetheless performs an important function, bridging the gap between the story's moment of highest emotional intensity (the climax) and the final end of the conflict that has driven the story (the resolution).
In this text, we can see that the falling action therefore occurs after Dean abandons Sal in Mexico City when he is sick with dysentry. Sal is very upset by his supposed friend's desertion of him, and his realisation that Dean is a "rat" for leaving him when he was so sick to go and chase his own selfish desires marks the falling action, as Sal is forced to reassess his own admiration and respect for Dean and to realise that he follows the kind of life that is built on no commitment and a child-like shunning of responsibility and adulthood.
The falling action thus includes Sal's return to the US and his chance meeting in Texas with an old man who offers him the following sage advice: "Go moan for man." Sal returns to New York and meets his true love, Laura. Sal therefore begins to move away from Dean in embarking upon a serious, committed relationship. The resolution features his final meeting with Dean and his realisation of how different they have become.