Her home city of Hanoi doesn't have tap water that the locals like to drink. Even after boiling I think it can taste bad. So she grew up with heavily filtered water or bottled water. I would guess her diet was lacking in many trace minerals from this.
Just because a treatment works doesn't mean it's the best treatment, or is treating the "cause." To say that a lack of fluoride is the cause of their bad teeth is silly. Not eating sugar/acidic foods, Novamin, or a different set of bacteria in her mouth would also improve her teeth.
As with many things genetics plays a big part.
I neglected going to the dentist for 15 years and take just ok care of my teeth (brush at least once per day, don't floss).
I was expecting bad news, lots of cavities, but my teeth are perfect. After some plaque removal, not a single cavity.
Asked the dentist about this, he shrugged and just said "genetics".