Emission equipment is covered under an 8 yr 80K warranty. This law was put in place in March 1996 and hasn't changed since. The manufactures and dealers have no say in this matter. If they refuse to repair the car simply call you local emission testing center and hey will give you the number to call to report a dealer.
What you are smelling was most likely a bad cat but those problems typically don't go away on their own. Being a 2008 model their will be a sensor before and after the cat monitoring exhaust. If there is an issue it should trip the check engine light on the dash.
The next time the car smells like rotten eggs drive straight to the dealer. They'll know why you're there the moment you pull in by the smell