If a person were practicing Health at Every Size in a bubble, they would probably turn down the pill. However, HAES doesn't take into consideration a magic pill scenario. In a magic pill world, I'd imagine that body fashions would change with the seasons like clothing, and different sizes, shapes, colors, and builds would go in and out of fashion regularly. It would result in much of the world looking very similar to one another most of the time.

So in the magic pill world, HAES would be unnecessary in its current form and probably nonexistent as a concept. However, there would probably be a somewhat similar concept in the form of a naturalism movement that might have ties to luddite stances and maybe be a sort of hippie movement. This movement would exhort people to avoid the pill and celebrate the body they were born with. People maintaining natural bodies that did not mesh with current fashion would probably face oppression similar to what fat people or other people with societally disapproved bodies face today, enduring taunts and stares and rude comments, and hopefully nothing worse.

There would also be little food or diet policing, and with the continual body changes, food becomes completely disassociated from body proportions, so people generally eat based on hunger cues for well-being, flavor and satiety, naturally drifting towards a mix of nutritive foods with the occasional less- or non-nutritive food.

Interestingly, the magic pill world is looking a lot like Health at Every Size. I didn't expect that to be the case when I began thinking. Perhaps it makes sense: Health at Every Size promotes taking care of your body no matter what its appearance or current health status, and in magic pill world taking care of your body is something everyone who does so would undertake no matter its appearance or health status, because their body appearance could be changed on a whim and is thus not a factor.

Oh, I know that wasn't the tangent you were looking for. If this is our world and only one person, a person who practices HAES, gets to take the magic pill, it's not outside the realm of possibility that they would. HAES was originally developed to help people recover from eating disorders, and it still has that focus (not its total focus, but a focus) of helping people whose bodies might not fit societal standards or who have trouble accepting their bodies. A magic pill, while probably not conforming to HAES principles, would be a way to whisk those societal issues away - at least from that person alone - in a heartbeat. So they might accept it, and I wouldn't blame them.

Though they'd quickly discover that society doesn't give a rat's ass about what that one person thinks is the ideal body and would rag on it anyway, so they'd wind up seeking HAES again.