I agree with gcathie. An older dog often does things that are unexpected and out of character. I'm sure they wouldn't have locked him in the room knowing that he would destroy anything. And afterwards, they were ashamed to show you the damage. Your son and DIL were still keeping the quilt on the bed, so it seems that they did cherish it, even after it was damaged. Talk to your son, but with love rather than resentment. Look at this as a great opportunity to make another quilt. It will strenghen the bonds between you and your son and DIL.