Some ideas about alternatives to "regular" cake:
Try something savory like cheese rolls or pizza muffins
If possible, use whole-grain flour or substitute part of the wheat with spelt flour. That loweres the glycemic index of the food (blood-sugar rises slower).
Make fingerfood. That makes it easier for your friend to eat just a tiny amount while the others can take several pieces.
Fresh or dried fruit and dark chokolate are generally safe for diabetics. They combine very well ;)
That way you can celebrate with tasty food and the diabetic does not feel left out.
Edit 1: in response to several comments and downvotes I removed the substitution of glucose sugar from the list of alternate solutions.
Edit 2: Nowhere in my answer do I suggest hiding any nutritional information or force-feeding anyone. I merely listed possible alternate food items that anyone is free to eat or decline. Since the OP doesn't state (and possibly doesn't know) what type of diabetes their friend has, I still think my answer offers valid information. Of course, the OP should ask her friend for preferences. I just didn't want to repeat information given in several other answers.