Not to try and discredit your or your boss or anything but a lot of people assume that they got salmonella from the last food they ate. Usually it’s from food they ate a day or so ago, as the incubation time is between 24 and 48 hours.
If he got sick right after eating McDonald’s food then it’s probably just a coincidence. Have he got sick 2 or 3 days later then yeah it was probably the McDonald’s burger.