My sympathy to the friends and family of the lad.
With almost all hobbies and pets there is a risk of unexpected tragedies; from time to time one reads of dog attacks and people in hovercraft being overcome by eels.
In my experience it is almost unknown for fancy rats to bite of they are treated with a degree of knowledge and respect*. I would have that the risk is more of being licked half to death. As I say a terrible shame but I think it is not an inevitable result of keeping rats.
* I am not suggesting for a moment the dead boy didn't handle them properly.