How much sugar should a prediabetic have?

Few to NONE.

You can help to prevent Diabetes and heart attacks by AVOIDING sugar, wheat, cereals, and other refined/processed carbohydrates, and exercising, will help you to avoid moving into diabetes.

A LIFE STYLE change is called for - Sugar and all other refined/processed carbohydrates are the ENEMY. Lose weight. A Body mass Index of 25 or lower will put you in great shape.

Exercise - As little as 30 minutes cardio walking every day. Bad weather outdoors? - Walk up and down a staircase for an exercise routine, and do it up to 30 minutes every day. Better still, if you can afford to, buy a good treadmill.