I think she should contact her doctor/specialist for some information to help understand what is going on. Here are some links that may be helpful to you and your step-daughter. I wish you both the best.
i heard of it but never looked it up
Functional Immotility isn't the same as gastroparesis.
The treatment of such can be combining sugar with white vinegar at a rate of half-and-half or related to such treatment. The effective dosage for an average human (150cm by 45cm by 70cm) is around 5L per day. That is, if it is only gastroparesis with some diagnostics of functional immotility.