A good quality whey protein isolate is perfect fine for a diabetic to use. You just need to make sure to be using a quality product that isn't full of fillers. A lot of supplement companies cut their protein with cheap carb fillers like maltodextrin to reduce the cost of the product.
All protein powders are not created equal. You really need to be a smart consumer in today's market and do your research. Look at the ingredient list and supplements fact panel on every supplement before buying it.
For a whey protein - you want to see nothing but that in the ingredients list. Whey protein, natural flavours, a natural calorie free sweetener like stevia and then maybe some salt, soy lechitin and cocoa powder. The board sponsor n2bm.com makes a really good product called wheytobuildmuscle. Check that out.