There is a lot of flour (starch= carbs) in cake
It's not just the sugar. If the bride and groom would like to accomodate their diabetic guests, suggest fruit alternatives. Some diabetics can have a piece of cake and if and when they do, prefer just a bite of the real thing, as opposed to a big piece of diabetic dessert substitute. Others forego it altogether. Most adult diabetics can control their diabetes quite well, it is not the cake makers responsibility, not the bride and grooms. Just make sure you let them sign off on the cake, that cake is not a food for diabetics, it is a normal cake.
I would not go the route to make them a diabetic cake, if you do not have the experience. I made on for a diabetic once (Hazelnut cake, no flour, minimal sugar) Was fondant covered, that he could peel off, BUT they had to sign for it. I also gave them a complete list (incl weights) of ingredients in the whole cake, which were sugar and starch based, so that they could count out the points