In my opinion, there are two possible themes to the story.
The first is that two people who are from the same place and who are good friends can still grow apart. This implies that some people are just born to be good and others to end up badly. Jimmy Wells stayed honest, but Bob didn't.
The other theme is that morality and duty are more important than friendship. Jimmy clearly still feels something for Bob -- he can't bring himself to actually arrest Bob. But he knows that doing what's right is more important than friendship.
The story "After Twenty Years" by O'Henry is a story about a cop and man who meet on the street. The man tells the cop that he is there to see a friend that he has not seen in twenty years. Years before, he and his friend had agreed to meet at the very spot discuss how their life had turned out. The man goes on to tell the cop how his life had been and that he had been successful. He puts down the other man and says that in the past:
"He was a kind of plodder, though, good fellow as he was. I've had to compete with some of the sharpest wits going to get my pile."
The cop dismisses himself. Shortly, a man comes to greet the man. He tells him that he is his friend, but the man realizes that the man is too tall to have been his friend. The new man arrests the waiting man. The man then gives the waiting man a note.
It turns out that the waiting man's friend had actually been the policeman who could not arrest his friend, but had realized that his friend was a man wanted by the law when he had it a match to light his cigarette.
The theme in the story is about loyalty versus the law.
I also believe the theme is about choices. One chose to follow a good path by working on the side of the law and the other chose to follow a bad path by breaking the law.
$)Itis important to do the right thing—even if it is hard.
$)Time changes people