Leslie was a better friend than Jess, do you agree with this statement?

This is a standard "state and defend what you think" writing prompt. There isn't a one correct answer. It is up to individual readers to answer true or false and then defend that particular answer. I'm not sure if you have to answer this question in a paragraph or in a full essay. If it is an essay, then I would recommend answering this question with a double sided thesis statement. This will allow you to discuss both answers to the question. For example: "Although Leslie and Jess appear to be friends of equal merit, Leslie is actually the better friend."
This kind of thesis setup will allow you to discuss when, where, and why Jess and Leslie are equal friends to each other. Your main task is then to prove why that logic isn't quite right and prove that Leslie (or Jess if you reverse the thesis statement) is the better friend.
As for what I think personally, I do not see Leslie or Jess as being a better friend than the other. Neither of them is perfect and their friendship does go through growing pains. You could argue that Jess' move to go to the museum with Miss Edmunds instead of hanging out with Leslie was a selfish move, but I don't believe that Jess went to the museum to intentionally hurt Leslie. He felt guilty for not asking if Leslie could come. He is glad that he gets to be with Miss Edmunds alone, but that doesn't make him a less worthy friend. Leslie and Jess both care deeply for each other, and they show their support for one another throughout the story. The Christmas present exchange is a good example. Their gifts to each other are incredibly thoughtful and show a real knowledge of the other person.
Perhaps you could argue that Leslie is a better friend because she is more willing and open to being a friend in the first place. Jess initially resists associating with Leslie because she's new and she beat him at the running races. His pride is hurt but I don't think that makes him a rotten friend. They weren't friends at that point.