Well I would probably move her if it was me. Horses, especially ponies, don't need grain supplement unless they are in heavy work. You can find safer alternatives to put weight on an undernourished horse.

Sounds to me like you need to make sure your pony is wormed and kept separately from the rest of the herd at feeding time. She may be being pushed away from the food if she's not the dominant horse and they are in a group setting.

If she's already kept separately than there is something else wrong, such as worms or not enough being fed. Ponies are generally very hardy and rarely need the kind of upkeep a larger horse can sometimes need.

Ideally, she should be kept separately and free fed hay all day long with a weight gain supplement until she starts to pick back up. If she cannot be free fed then she should be kept separately and feed three times a day with supplemental feeding of a complete feed like SafeChoice by Nutrena or Purina Strategy, rather than a grain which coudl cause laminitis.

I used a product called "weight gain" to put weight on my walker when i got him as he was quite thin, along with the Safe Choice.

If those things can't be done for you where she is now, you should try to find a new place to keep her. Good luck with her.