take water recomendations seriously, but with a grain of salt. your friend could also be experiencing food poisoning from under-cooked food, alcohol withdrawal/over-indulgence, or the simple dangers of having a little too much fun of the socially unacceptable to admit to variety. the previous poster is absolutely right about particular bacteria in the water of particular places, but unless you're prone to illness or gastrointestinal issues, don't worry too much about drinking coke. if you're gonna go that far, you need to not eat anything that hasn't been boiled either. as most utensils and uncooked foods are rinsed in that self-same water. or brush your teeth with anything other than bottled water. or take a shower, as you might get some water in your mouth or ears, etc. really, unless there's a water warning issued for the area, if you're going to be that picky, you might consider only traveling to 1st world nations. most younger travelers are more likely to give themselves diahrrea from drinking too much, than from drinking local coke.
(ps- coke produced in mexico is made with real sugar rather than high fructose corn syrup -which makes it less carcinogenic! it sells like hotcakes in US border cities in the southwest as a result.)