Yeah, she's pretty thin. I'd hold off on riding her any more until she has any kind of butt muscles, right now she doesn't.
Best formula for weight gain is as much hay as she can eat, great grass, and free choice minerals and salt. You can get the minerals and salt from a feed store, loose, as well as feeders you can' screw into a stall wall that will hold a few scoops of both. I'm sure she'll devour all of the minerals you put into the feeder completely for about a week, but that's okay. I'm sure she's very deficient, and it won't make her sick if she eats a ton of them. You'll know she doesn't need constant refills when you go to fill the salt/minerals and there are some left from the day before. She'll eat only as much as she needs.
I wouldn't do sweet feed, but that's just me. I refuse to feed any type of sweet feed, an underweight horse doesn't need the calories from sugar and molasses, they need the forage and the healthy carbs.
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