I would be willing to do almost anything to make Art happy. I care about our friendship. The only thing I won't do is change the essence of my work.

This quote means that the person is willing to do anything to make their friend, Art, happy. This can include an array of actions such as spending more time together, going out of one’s way to help solve problems, or even listening intently. Regardless of what the person does, though, the one thing they won’t do is change their work. This implies that their work has a certain essence, or meaning, that cannot be altered for anyone. Therefore, the person cares deeply about Art and their friendship, but won’t compromise the value of their work.