Can my dad use protein powder in his shakes if he is diabetic? He likes the idea that it will give him "strength". Can he take whey protein?

Yes.. Keep an eye on the sugar and carbohydrate levels in his shake, and monitor glucose until you can see what effect the shakes might have. He may need to adjust your diet or your medications, depending on findings. This assumes, of course, that he has adequate kidney function.. Best Wishes !
Protein is OK. The protein will be fine. It will not spike his fingersticks. As long as there isn't something else in his shakes or whey that could harm him or raise his glucose levels. In fact, a protein and non-starchy vegetable diet can help diabetics control their glucose levels.