Be careful what you give her at first. I mean supplements, hay... whatever... ANY of that can make her gravely ill right now if she's bad off. How bad is she? Do you have any pictures?
Depending on how bad she is... she should probably get nothing but water and hay, in small amounts several time a day. I wouldn't start her on any rich hay either (First cuttings, alfalfa, etc). Her gut flora is going to be crappy, and she will be prone to gorging and that could lead to colic or foundering. You may also have food aggression to deal with - sometimes a starved horse (or dog) never gets over that and will be 'grabby' or especially greedy for the rest of their lives. If she's a body condition low 2 or less,
I would have a vet on board the moment she got to my place.... because she may not even be healthy enough to be wormed. IDK. I'd want a vet's opinion immediately.
If she's just kinda ribby and needing some groceries on her, but not emaciated (mid-2/3 Body Condition - see chart below), I'd start with hay (Not straw - straw is for bedding!) for the first week or two, then gradually introduce a good quality pelleted senior feed (Yes, I know, probably not a senior horse but it's easy on their stomach, easy to chew, has a good nutritional content) (NOT GRAIN!) in small quantities along with increasing the hay. You can introduce a free choice horse formulated 'protein' bucket to help pack on the pounds and nutrition too - I keep one out for our senior horse who sometimes hovers between a 2 and a 3, and I have a couple of friends who have worked with local law enforcement and have fostered horses who were taken out of abusive homes and were malnourished. They swear by these tubs
and so do I. Once you're out of the woods on any health or behavioral blow back from too long without regular meals, you can introduce rice bran (fat) into the diet. I would be sure to provide horse mineral and salts, free choice... and work your way up to letting the horse free choice hay all day, every day with feedings of the senior feed and rice bran twice a day,. Maybe even some fine gauge alfalfa pellets (But introduce them gradually - too much too soon and you'll have a horse with the drizzling poops). I'd go ahead and worm her as you plan to. Once she's in the clear as far as weight goes, you can gradually shift her to a non-senior feed. Keep out free choice hay if the weather is cold right now or the grazing isn't great where you live in the spring/summer/autumn.
For a truly emaciated horse, at a body condition 1, I strongly recommend you consult an equine vet and follow their directions before you worm them or try to fatten them back up yourself. The organs can be damaged, you could have all kinds of issues that you might not consider right off. Better to err on the side of caution when dealing with a low weight horse.... even Superman, our own senior horse, when he dropped a shocking amount of weight two summers ago? We sought the advice of our equine vet, just to be safe.