Well he created a giant Wedding Cake, a food item that a chef would reasonably expect to be shared by a large group of people over an extended period of time, not devoured by a lone individual in single sitting.
Big Mom eating the whole thing would be 100% on her and I would declare Sanji to be free of any liability.
This comment made me realize how much I want a spinoff involving Sanji teaching Big Mom cooking/eating etiquette.
Sanji: "Make sure to eat all your food, and leave no leftovers, even if some gets on the ground."
Big Mom: "Ooops, I dropped a meatball, and some of the sauce stained the ground."
Sanji: "Eat the meatball, and I'll clean up the stain-"
Big Mom starts eating the floor itself to clean up the stain
Sanji: "...Yep, your manners are definitely improving, Linlin!"