Wow! I relate to your concerns ... coffee and diet coke are soooo acid! She is in real trouble doing this. Plus the sweeteners lately are accused to actually put weight on, not like some of them are 0 calories. Plus brain damage from MSG, aspartame, sodium benzoate and what not. She would be so much better of with water sweetened with honey, and or lemon in it ...
Citation from wikipedia:
"The sodium benzoate was found to break down mitochondrial DNA in living yeast cells"
link: http://en.wikipedia.org/wiki/Diet_Coke