Bingo. This is an opportunity for Sarah to explain to Trig that animals are capable of feeling pain and may *hide pain* to keep from showing weakness.
If he is literally mentally incapable of understanding that one does not treat living creatures like inanimate objects, he is not mentally capable of being given unsupervised access to the family pets (which he clearly had for him to step onto the dog’s back).
Also, since she knows that Trig likes to help with the dishes, why isn’t there a stepping stool by the sink?
Actually it’s an opportunity for you to learn that how another parent parents is none of your concern.
But the animal was not in pain. Look at the photo: the dog is resting comfortably on the floor. Dogs don’t act macho and “hide pain.” They scream unendurably if in any kind of pain.
It’s not just a family pet, it is a highly trained dog, trained by a place that trains service dogs.