@chrisatl8: When one is diagnosed with diabetes, his/her cells are still perfectly capable of metabolizing glucose molecules. My understanding of diabetes is that the body cannot recognize glucose in the bloodstream (the reason depending on the type of diabetes). When blood sugar levels are high, the pancreas normally secretes insulin, which allows the glucose to exit the bloodstream and enter the cells. Your friend may not be creating insulin at all (or too little of it, as in type I), or perhaps the insulin created is not recognized by the body (which can occur upon frequent insulin releases, in the case of type II). Perhaps that will help?
Your friend should stay away from foods which are high in simple sugars, which do not need to be metabolized by enzymes (hydrolysis reactions) to be consumed by cells. Complex sugars which are linked together in polysaccharides are broken down over time, keeping blood sugar levels from dramatically spiking.