It's sad you feel that way about him.
With limited access to his usual routines, job, and responsibilities, exercise became one of the few activities that allowed him to feel a sense of independence.
He had an extremely traumatic episode and suffered brain problems. He may not even be aware of the loving care his wife is providing him and it's hardly "self-indulgent." Being outdoors in the fresh air has benefits far greater than being indoors on a treadmill.
It’s seems strange that people here at FR would criticize the wife or the husband for their choice and go beyond that by calling them selfish and narcissistic. That is their choice. It doesn’t affect any of us.