how do I stop my mom?

Jo
My mom keeps trying to give my almost 4 month old water whenever she watches him. She says it's to wash out the spit up taste in his mouth. I don't know how to stop her. She says he just loves it. I don't want him having water but she won't listen to me. I tell her the risks and she just brushes them off. How can I stop her from doing it? 
Update*** I took my mom with me to one of my son's doctor appointments and the doctor explained why it's not a good idea to give water to such a young baby. She apologized to me for giving him water and will now listen to my wishes.