Hello. I'm sorry that your niece is ill.
Diarrhea and stomach pain are most commonly signs of an infection of the gastrointestinal system. The source could be a virus (most common), a bacteria or parasite causing the symptoms. During such infections, a person is temporarily lactose intolerant. As you mentioned that she is consuming dairy, this is probably prolonging and exacerbating her diarrhea and stomach pain.
I would recommend withholding all dairy until the diarrhea stops. The electrolyte solution her pediatrician gave will help replace the electrolytes she is losing in the diarrhea, but it does not stop it. I would suggest that her mom continue to offer her solid foods as this will start to solidify her stool. If your niece insists on drinking milk, I would change to a lactose-free version or soy milk for now.
If the diarrhea persists despite these dietary interventions, lasting 7 days or more, she should have stool testing for bacterial or parasitic infection. Results of such testing will aid in determining if an antibiotic or anti-parasitic medication is needed. In general, no medication is recommended for stopping diarrhea in children, and it can actually make them more ill.
Hope this helps and that your niece feels better soon!
Thank you I will tel my sister this and if it help