@Amber221
Unless the parking spot was a secure location on your private property, of course you are not responsible. You shouldn't apologize or be responsible for something you have no control over. If the car was broken into, the guest should be contacting his auto insurance. If someone parked their car on the street in front of their hotel or a resturant and the car was broken into, would people expect the hotel or resturant to take responsibility?
Better yet, tell the guest to contact Airbnb CS 🙂