I’m sure he would understand if you told him you just weren’t comfortable with doing it BUT If it was me I would tell him that I was INCREDIBLY behind on school assignments and that with midterms coming up and that i don’t have enough time to study. Acting sick is also easy but with this excuse you could give him more time to find a substitute. If you tell him you can’t do it the day of it will make him a whole lot more disappointed than if you blamed it on school work a week in advance
Nick begins to realize that their world revolves around their money and his attitude begins to change into feeling bad for Gatsby and refecting the others