I believe that this question is asking about the sequence that occurs in chapter 18. This is the chapter that has Beauty running heroically hard to fetch the doctor.
Beauty's efforts were only meant to be one-way, but none of the doctor's horses are available for the doctor to ride to Birtwick Park. This means that Beauty is forced to carry the doctor, an inexperienced rider, back to Birtwick Park. Beauty arrives at home completely exhausted. He has practically run himself to death, and Joe leads Beauty to his stable.
Readers are told that Beauty is shaking with exhaustion and that every hair on his body is soaked with sweat. It steams off of his body, and Joe thinks that Beauty is too hot to be covered with a blanket. Joe also thinks that Beauty needs to rehydrate, so Joe brings Beauty a bucket of cold water along with some hay and corn. Joe then leaves Beauty.
Beauty drinks all of the water and soon after begins to shake and turn cold. Joe's error in horse care causes Beauty to get very sick, and only through the vigilant and continuous efforts of John is Beauty able to be nursed back to health. John knows that Joe didn't know any better, but John is unable to speak kindly to Joe in the days that follow.