Don't deworm her until the vet looks at her, then buy whatever the vet recommends.
For feed, right now I would be giving her small meals of grass hay 4-6 times a day. Add to that some alfalfa, either hay or pellets. Just 1 lb per feeding (NOT MUCH!). Once she's doing well for 3-4 days, start increasing the amount you're feeding, slowly over two weeks. You want her to work up to free choice grass hay and 6-8 lbs of alfalfa pellets or hay broken up into 2-4 meals.
Alfalfa is EXCELLENT at putting on weight without risking colic or founder.
Please do not feed yogurt. The equine digestive tract is not set up to digest dairy products, or any animal protein. Once a day, give her a small meal of hay pellets, some vitamins, and a probiotic (like fastrack or source focus SR). For vitamins, I recommend NutriPlus++
(the 1oz serving) or even better, Platinum Performance
. PP has probiotics, joint goodies, and everything else she'll need. Start her on a half dose for a week, then give her the full recommended dosage.
Using alfalfa pellets as her "feed" will help slowly return her metabolism to normal. Horse feed pellets are usually high in starch and sugar, which is not good for weight gain on an emaciated horse. UC Davis did a study that found Alfalfa to be the best and safest way to put weight on a horse. Your BO can add some water to the pellets, just enough to coat them, and let them set for about 5 minutes. The alfalfa pellets will start to break up pretty quickly and turn "fluffy".
Be sure the vet checks her teeth and pulls blood, to check for any illness that could be causing her weight issue.
Don't forget to take pictures! They will help you document her progress. Take photos and weight tape measurements once a week, on the same day at the same time of day, to make sure you get comparable results.