What are your winters like? Rainy or snowy mostly?
Horses seem to do better in snow than in rain. The snow can sort of sit on top of the coat, but rain tends to soak through and they can get really cold.
The other issue is the run-in shelter; does your horse have to share it or does she have her own? If she is sharing the shelter, how many horses have to use the same one? Many times I have seen one horse be in a shelter and not let any other horses in.
I agree with this. Yesterday, we had crazy weather. It rained and went up to 17C in the afternoon, then the temps plumeted to below freezing. I blanketed both horses, which is rare for me! But a) we've had a really warm fall so they're not used to the cold yet and don't have a thick winter coat. b) they were very wet from being rained on all day, and that water was going to freeze on them, chilling them to the bone. Tonight, the temps are just as cold, but I didn't blanket because it's a dry cold.
I have often seen my mare with a crust of snow on her, and she couldn't care less. She will stand out in a blizzard. My Arab gelding on the other hand, barely pokes his head out of his stall door (their stalls stay open so they can come in and out as they please). Every horse is different, and conditions vary so much. Wet cold is much worse than dry cold, or even snow. You really have to monitor the situation and adapt accordingly. As
@AnitaAnne says, watch the other horses, if any, to make sure they are allowing your horse in the shelter. Same with food and water. The new horse is often the odd one out.
As for being out 24/7, I don't know any horses who would have a problem with that. It's how horses were meant to live in my opinion.