A horse that is not eating is not a horse that has any quality of life. They stop eating as their bodily processes shut down, which is, it should go without saying, more often than not an excrutiatingly painful process. There's a reason that the cessation of food consumption is generally given as the guideline for when euthanasia is justified. I can't believe that the barn owner or other boarders have not attempted or suggested the option of the relief of suffering....I can't imagine if it's a case of head-in-the-clouds or what.
I have not burned a carcass, but have heard it said that it takes an extreme amount of wood, far more than you would expect, and of course is extremely hot, and smelly, and releases toxins and organic material as airborne particles. The fact that a dead horse is being burned is not what will upset the other horses, but the sky-high bonfire sure might, in addition to being dangerous and, from the sounds of your burn ban, illegal.
The price of hauling carcasses off varies from location to location. Some areas, I hear, do not even offer the service. Here, the rendering plant does pick-up for around $100. And the local dump will accept animal remains, as well, if you bring it to them.