I'd be the last person to defend McDonald's but it's really not very surprising. Once food is cooked, it is - briefly, at any rate - free of almost all bacteria. If that woman's office was kept at a fairly consistent temperature and the atmosphere was dry enough, and the burger wasn't handled too much thereby exposing it to too many bacteria, the food is going to hold up for a while, a long while evidently.

Other factors include the sugars in the French fries. McDonald's uses very specific strains of Russet potatoes, which are among the starchiest of potatoes and the starches convert to sugars once cooked. Sugar, of course, is a great preservative (besides the flavor it adds to food, that's one of the major reasons sugar in its various forms turns up in so many processed foods nowadays).

The ketchup and mustard on the burger, too, are made with preservatives such as sugar and vinegar - in the case of the ketchup - and yet more vinegar, in the case of the mustard.

It was a joke for April Fools Day. Did you not get that?