Yes, certainly.
If that patient has completed an advance request for medical assistance in dying to avoid being in the kind of situation you described, then that patient is certainly suffering. If the conditions have been specified and the patient has reached that point, then we go ahead with medical assistance in dying, in accordance with that patient's wishes as specified in the advance request.
No one can say that patients, just because they can bop in time to the music, are happy. There is no such thing as a happy form of dementia; dementia exists and manifests itself in different ways. A person who was capable at the time they specified details in their advance request and who gave their free and informed consent does not want to experience that kind of situation.