If they were paid the then-current fair market value of the motorcycle, you are not responsible for the remaining balance on the motorcycle. When a vehicle is damaged or destroyed by someone, the owner is entitled to what the vehicle is then worth, which may be less than they paid or owed (e.g. if the financed it) or more (if it was a gift, for example). They are only entitled to the motorcycle's "blue book" value, and if they wanted to be protected against having to pay the remaining balance on a totalled motorcycle, they should have  purchased "Gap" insurance.