Some things that could happen are
diabetic coma, the inability to contact 911 or administer
glucose in some fashion.
To answer question 2, its really different for everyone. Normal blood sugars should range between 70 and 120. I do know that coma, brain dmg and death can occur when blood sugars go to low. That number is different for everyone.
Here are some suggestions, always remember, dont guess, test. Testing is the initial format for your father to know where is at.