I'm sorry to be so 'in-sensitive' but did this family ever brush their teeth? Adolescents don't get six rotted teeth without total disregard for dental hygiene and a very unhealthy diet.
Perhaps this family has something congenitally which makes them more susceptible than most to tooth decay.
That said, I was always amazed at the number of high schoolers who seemed to have a steady diet of candy, soda and white flour items like bagels who also had no cavities. I attributed it to fluoride in the water and possibly dental treatments when young. I wonder about their future gum problems, but they were certainly way behind me in getting cavities.