Do you mean that you found the 36mm large axel nut laying in your garage and that your front axel was then loose (which it would be) or the axel was broken?
If that's true, then I'd find that very suspicious as even if it wasn't tight enough 2 months ago, it's not likely that it'd just unscrew conveniently in your garage vs fall off on the road somewhere.

Before accusing your dealer for this, realize that even if they failed to properly tighten it (if previously removed?), it would have become a noticeable problem soon after leaving the dealer, not 2 months later. I have also seen cars where those nuts can and do loosen by themselves (not common however) but the noise that causes usually becomes obvious long before anything is damaged, it doesn't just spin off quickly in a day or two, it takes more time than that and there's obvious noise from that wheel too.