I am not an expert but from the post mortem by Samsung I read there was a physical flaw with the battery which when breached by a fluke fall or drop or even squeeze created a set of circumstances that caused the failure.
You don't even need to be charging it to trigger the catastrophe.
I don't want to get all moral on you but the responsible thing to do would be to get that thing back to Samsung and out of the wild. I know it's tough I had that device as well