Girls, there's a question. My little one recently got sick with a rota…

Girls, there's a question. My little one recently got sick with a rotavirus, many of us here have been infected, especially the kids. So, here's the question: my daughter was constantly vomiting already just with water, I called the doctor, she said to call an ambulance so they could give her ceruical injection, otherwise it would lead to dehydration, so I did that, and then my relatives started saying that this injection can't be done, only in emergency cases, that it gets everything unnecessary out of the body, but my daughter was already struggling to vomit, she was suffering so much. What is your opinion about this injection and when should it be given, when not needed?