I’m going to go out on a limb and say that this letter is 100% real. It’s too mean not to be.
With less than complete certitude, I’m also going to guess the author of the letter is also obese. Why? Because the only person in their right fucking mind who would admonish an overweight person to stop actively trying to lose weight is another fat person who doesn’t have the fortitude to make their own life better. They can’t stand looking out the window in their kitchen, as they patiently wait for their seventh Hot Pocket of the day to cool, only to see their neighbor working hard to make a transformation. The only other possibility is that the author is just a lifetime cunt, because after a second time reading through it, this letter reads like a mean girl from high school who never stopped being mean.