While only a minor character in an early chapter in the book, Billie Jo's best friend, Livie, plays an important role in foreshadowing the hard times to come for the town. Her family goes to California, and while no reasons for leaving are given at first, we eventually come to understand that they're going to find work after the drought's devastation of the land.
They don't have too much luck, though, as Billie Jo later gets a letter from Livie saying that her older brother left the family to go find work on his own. And when Billie Jo makes her brief railroad escape from home, she thinks she sees Livie through the slats in the boxcar—and not looking good.
Billie Jo has a lot of her friends leave home throughout the book, but Livie, being the first, carries special significance for the rest of the story. At first Billie Jo is jealous that Livie gets to leave home (3.6), but Livie's family's story gradually reveals that leaving isn't necessarily the right solution. The lack of work in California splits up her family, and the possibility that Billie Jo saw her from the boxcar indicates that she herself ended up separated from them.
We can't say for sure, but it's possible that seeing Livie through the boxcar plays an important part in Billie Jo's decision to go back home.