I've had old hens just as you describe here. They get to the age where they develop heart disease, have breathing issues, become thin, arthritic, no balance,.can't roost and have to sleep on the floor.
Old hens need supplemental heat on cold nights as they truly can freeze to death and or have a heart attack from the cold. I keep my old hens caged at night so I can use blankets and extra heat. For mildly cold nights, I use a panel heater (150 watts) and set it on bricks against the side of the cage. With an old quilt, I cover all sides and most of the top, except the side with the panel heater, and this creates a warm room. If it's really going to get cold, 5 or below, I use an infra red lamp on the outside of the cage, sides only of the cage covered with the quilt.
If you can't do either of these, you should definitely bring her inside at night, or at least into a warmer building or garage. Old birds just can't tolerate the cold without dying, especially heart patients as your girl probably is.