Can you put her in the paddock with a friend, and their own round bale?
For feed, I would probably go with senior feed and add 1-2 cups of flax meal. Start with just 1 scoop of senior feed a day with 1 cup of flax, then after a week increase it to 2 scoops of senior and 1.5 cups of flax. For the third week, increase it to 3 scoops of senior and 2 cups of flax. If she won't eat all the senior feed, feed her as much as she will eat.
I'd have her teeth checked too. Senior horses often need more frequent dental care. Yes, I know I just poo-poo'ed senior feed on another thread, but to keep things simple for this owner and horse, senior feed is probably the best way to go .
If the senior feed doesn't seem to be helping after 30-days, I would switch to alfalfa pellets, flax, and a vit/min supplement, like GrandVite or NutriPlus++. You might need to add some water to the alfalfa pellets if she has a hard time chewing.
You could also add alfalfa pellets to the senior feed for extra calories and protein. They're easier to eat/chew that hay cubes.
Also, adding a winter blanket will help her keep her weight up. Just a medium weight blanket with a waterproof outer shell should be enough if she has a good winter coat already.