I have a mixed reaction to this. To the good, I’m glad they adopted the dog and that he has a loving home where he is appreciated. Being able to understand at least some sign language will be a useful thing. On the other hand, a dog can be so much more than a companion and a pet. The sense of hearing of a dog is so much more than that of humans, and this could be the difference between getting a crucial warning that something is awry or not. An approaching severe storm or tornado, someone trying to break into the house, a car roaring up out of control from behind them while on a walk, and so on.
Agreed—better and safer to match the unhearing with the hearing.