Who is legally responsible in the case of a fire if the owner was not living in the house, but a relative was?

My mother is 93 and living in Assissted Living. Her house burned. Her grandson was living in it at the time. Her neighbor's house was damaged by the fire. Their insurance company is asking to be reimbursed for damages from my mother. My mother had no insurance on her house. Can her grandson be required to pay the damages to the neighbor's insurance company?