why cant you drik pop or tea instead of water

You weren't advised that, she was. 16 years of drinking 2 litres of coke a day and nothing to flush it away cannot be good for you. She has been advised to drink water to try and clean up her system and if she enjoys 2 litres a day of sugary drink she probably wouldn't know when to stop if she drank any other replacement.
She has been told to drink water as part of her medical treatment, that doesn't mean you should. Coke won't kill you, alcohol won't kill you, nor will cigarettes or red meat or salt, unless you expose yourself to too much of them.
Everything in moderation,  sunshine, fresh air, and plenty of gentle exercise will keep you healthy.

If you are diarrhoeic, water with a little salt and some sugar or starch can be retained better than pure water.
If you diabetic, water with sugar can kill you, or save your life.
If you are dehydrated, not enough water, or water with too much salt can kill you.
If you have hyponatraemia, too much water, or water with not enough salt can kill you.
Too many calories or not enough calories can kill you.
It depends on the situation and your body is probably pretty good at telling you which situation you are in, but sometimes you can fool it with things it hasn't evolved to know how to handle.

The pure water is okay. What is missing was probably enough Potassium and Sodium to  keep things in balance. When my wife was a cancer patient taking chemo she was very sensitive to salt and ate almost none. Her electrolytes would get low. She would have to go to the hospital and get IV potassium and sodium. I started adding potassium chloride (lite salt) to her food to help keep the levels up to normal.

I noticed Coca-Cola used reverse osmosis to purify their water and added magnesium sulfate for flavor which removes the potassium and sodium. I just looked on a new bottle of Coca-Cola Dasanti water and it now has potassium chloride added back. I think the lady was also avoiding dark green leaf vegetables and banana which are a good source of potassium.

"non-sugar" still implies a sweetener (and what type of sugar is it not? not beet sugar? not cane sugar? not corn syrup?)

INGREDIENTS IN SOFT DRINKS-A WITCH'S BREW
High Fructose Corn Syrup, now used in preference to sugar, is associated with poor development of collagen in growing animals, especially in the context of copper deficiency. All fructose must be metabolized by the liver. Animals on high-fructose diets develop liver problems similar to those of alcoholics.

Aspartame, used in diet sodas, is a potent neurotoxin and endocrine disrupter.

Caffeine stimulates the adrenal gland without providing nourishment. In large amounts, caffeine can lead to adrenal exhaustion, especially in children.

Phosphoric acid, added to give soft drinks "bite," is associated with calcium loss.

Citric acid often contains traces of MSG, a neurotoxin.

Artificial Flavors may also contain traces of MSG.

Water may contain high amounts of fluoride and other contaminants.

Soft Drinks - America by Judith Valentine, PhD, CNA, CNC

Here the author offers a number of factors to worry about in soft drinks - and then compounds and confuses the issue with "'Water may contain high amounts of fluoride and other contaminants."

But: we actually need some of those "contaminants".

Thus, there still remain just 2 certainties in life: death and taxes.

You can always die in jail.