I think that you will have to focus on the last chapter of Steinbeck's work. Initially, it should be noted that I am not entirely certain that the ending of the novella totally supports the thesis statement. The death of Lennie is an indication that adversity necessitates what George must do. Friendship, in the most living of senses, does not triumph. Yet, I still think that there are some elements that you can pull from the last chapter to help you. For example, I think that you would have to focus on how Lennie's biggest fear in the last chapter is that George is going to leave him. The hallucination of the rabbit and Aunt Clara both strike at this fear. When Lennie engages in conversation with both hallucinations, the underlying element is this fear of being abandoned. When George appears, Lennie sees it as an affirmation of friendship over fear, of solidarity over alienation. Lennie is consistently reciting how "guys like them" remain unified as opposed to others who are separate and who drift from one another. If you wanted to pivot to the ending, I suppose a case can be made that it makes sense that George, as Lennie's only true friend, stand up for their friendship by doing what he needs to do. It is a statement of mercy from a friend, as opposed to Curley and the lynch mob's desire to cause harm to Lennie. In this, an element of friendship triumphs over its darker countervailing force. However, I think that your essay becomes stronger when you appropriate Lennie's point of view in how the story represents the triumph of friendship over adversity.