According to the Univ of Minn a horse should be very comfortable in 50 deg weather.
Cold temperatures will increase a horse's energy requirement as the need to maintain core body temperature increases. The temperature below which a horse needs additional energy to maintain body warmth is called the lower critical temperature. The lower critical temperature for a horse is estimated to be 41°F with a summer coat and 18°F with a winter coat (upper critical temperature is estimated at 86°F). However, the lower critical temperature can be affected by individual horse characteristics. A horse with short hair that is exposed to cold, wet weather will have a lower critical temperature higher than that of a horse with a thick hair coat and fat stores who is acclimated to cold weather. Another factor that can influence lower critical temperature is the size of the animal. Smaller animals have a greater surface area relative to body weight and can lose heat more rapidly than a larger animal. A weanling may reach their lower critical temperature before a mature horse. More importantly, cold weather can slow growth because calories are diverted from weight gain to temperature maintenance. To minimize a growth slump during cold weather, young horses should be fed additional calories.
Energy needs for a horse at maintenance increase about 1% for each degree below 18°F. For example, if the temperature is 0°F, a 1,000 pound idle, adult horse would need an approximately 2 additional pounds of forage daily. It is best to provide the extra energy as forage. Some believe that feeding more grain will help keep a horse warmer. However, not as much heat is produced as a by-product of digestion, absorption, and utilization of grain as is produced from the microbial fermentation of forage. Most data suggest that the need for other nutrients do not change during cold weather. However, consider feeding loose salt instead of block salt, as horses may not want to lick cold salt blocks during winter months.
During winter months, heavy hair coats can often hide weight loss. Regular body condition scoring is recommended to gauge weight and assess horse health. If a horse starts to lose body condition, increases in feed are recommended. Conversely, if a horse starts gaining excessive body condition, reducing the feed is necessary. Sorting horses by age, body condition, and nutrient requirements makes it easier to feed groups of horses appropriately.
Shelter
Dr. Krishona Martinson, University of Minnesota
Figure 1. Horses need access to shelter and should be fed additional hay during adverse winter weather. Tip: feed hay in a container or feeder to reduce waste
Horses should have access to shelter from wind, sleet, and storms (Figure 1). Free access to a stable or an open-sided shed works well, as do trees if a building is not available. In the absence of wind and moisture, horses tolerate temperatures at or slightly below 0°F. If horses have access to a shelter, they can tolerate temperatures as low as -40°F. However, horses are most comfortable at temperatures between 18 and 59°F, depending on their hair coat.
Maybe the vet needs an education on horses.