Well, you can tell her that many studies have proven that regular soda drinkers are more likely to be obese and that consuming high sugar drinks like soda can contribute to the onset diabetes.....
It is also interesting to note that sodas such as coke can be used to remove corrosion from metal surfaces (such as battery terminals) and glass.
( Try it on a bug coated windshield.) So, do you really want to consume anything that high in acid on a regular basis? Just my humble opinion.
When a person reaches beyond the limits of their ability; they shall they make progress. There is no other way.