I live in Georgia, so it doesn't get extremely cold, but I do blanket. My horse have 4 different types each and I will put different wieght blankets on them according to the temperature.
If it is 60-50 degrees, they wear a sheet. If it is 49-45, they wear a light wieght blanket. If it is 44-33 they wear a medium wieght blanket. If it is 32 degrees and under they wear their really heavy blankets.I also stall mine.
But, if this horse has a thin coat, I would definitely blanket her. I believe in blanketing every horse.