If someone has diabetes and fasts will they die?

Sure could.

If their BS drops too low, they can go into a diabetic coma and yes, they could die if they do not get some glucose in their system...I have seen people with their BS so low...that they would not wake up...no matter how hard you tried to wake them...But give them a bolus of glucose and they wake up instantly!!!

Diabetic people are not allowed to fast. Its could be harmful to their health

They might go into shock

They have to be careful anytime about their blood sugar levels.