Please, just tell the finger-licker why you won't eat a piece of cake she's cut. She probably doesn't even realize she's doing it or that it's grossing people out. She might even stop insisting on cutting every cake.
At my DS's scout banquet, the mom's had to cut the cakes for dessert...after cutting the first slice, we were all tempted to lick our fingers, like we would at home...I brought disinfecting wipes with me, carry them in my purse for shopping cart handles, etc. so we were able to clean up without putting our slobber all over the place. It was weird though, all the mom's, including me raised our hands about 4 inches toward our faces, then looked at the other moms and started to giggle. Sent our boys for the wipes and a big pile of napkins.