Have you tried her on a bland diet for a few days? It won’t help with the weight issue, but it will give her belly a chance to reset itself (assuming there’s no other underlying problem).
Then try adding a probiotic and/or Pepcid to her food. And feed small meals throughout the day.
Once she’s feeling better try a food you are comfortable with slowly.
She won’t gain weight if she isn’t keeping her food down or it’s coming out the other end.
Maybe consider something for her head. If she is really stressed she may need some psychopharmacuticals to get her through the anxiety so she can adjust to the changes.
