First off, start SLOW with ANY feed. I'm talking tiny portions, six times a day for ten days. Then four times a day for ten days. Then three times a day for seven days, and finally twice a day normal portions.
You can KILL her by feeding her if you are not VERY careful. In her condition, she probably would not survive a colic episode.
I would also give her a ProBios paste to aid in digestion. NO alfalfa at this point. A nice easy prairie or similar hay will do. Alfalfa can come in later to aid in weight gain.
You really also want to watch starches and sugars, as they will kill her. So no oats, corn, or sweet feed.
I personally find Purina Strategy to be a very good feed. I have been giving it to my own emancipated rescue gelding and he is now ready to be given alfalfa to further aid in weight gain. When I got my rescue, he was 1-1.2 on a Body Condition Score chart. This mare sounds like a 1. I would be extremely careful.
No wormer, vaccinations, or anything stressful at this point. You want to wait at least 60 days before even considering worming.
The stress of weaning could also quite possibly kill her at this point in time. I would start by working with the baby, getting him used to people and getting him off the mother as much as possible.
Wait! I'll fix it....