A very sad story and the loss of another great car. Its a difficult one to prove. The garage could claim that the driver caused the dislodge or damage which preceded the fire, highly unlikely, but something HRO will test.
They could also say that the fire was caused by a failed part not covered by the service.
At the end of the day, the car is insured but I get where the owner is coming from, and can see HRO having to pay out here to save face on their insurance, I am sure it will be done behind the scenes and HRO don't want to loose any more face here.