How common is it for waiters/waitresses to spit in peoples food?

i have this friend who whenever we go to some restaurant he like to argue the staff about anything. he'll call them back over and over again aggrevating them about anything. one time me him took our girlfriends to this really fancy restaurant and he called the waiter like 6 times to recook his steak and i knew for sure they spit on it or did something to it by then. i asked him why he did that and he said because hes paying good money and deserves to get the best for what he pays for. i asked hasn't ever seen those things on tv when people spit in your food and he just laughed. but i don't think its funny. i don't want to be caught in the crossfire of somebody trying to poison him but i just want to know and especially for people who've been waiters or waitresses if someone like that comes in on a scale from 1-10 1 being not likely 10 being very likely how common is it for them to spit in peoples food?