I'm not that convinced it's anything to worry about - I think a lot of things would last and look unmouldy if it was dehydrated and quite heavily salted too...
Having said that, McDonalds is rubbish food, but I don't think the fact it lasts without mildewing is at all evidence of that.
The woman claims that 'real food breaks down... this can't be real food'... that is clearly not true: dried fruit, rice, pasta, salami's, loads of foods that are dehydrated, basically, last for a very, very long time without changing in appearance. I even found some year old beetroots in the back of a cupboard last week (yuk, I know) - it hadn't gone at all mouldy, just dried out!
I suspect the burger has been dehydrated at some point.