Yesterday Somebody Hit My Truck...?

It was parked up on private property and somebody backed into the side panel of the truck bed and crushed it quite badly. I have their insurance details but I've never dealt with anything like this before and I was wondering if her insurance company can decide to write it off rather than fix it and I'm worried because the KBB value is less than I owe the finance company for the truck. It's a 1993 GMC Sonoma with 109k on the odometer and I also don't want to lose it because it runs really well.