Would my mother—who is 55 years old—be sinful if she does not wear the niqab, knowing that she has a heart condition and wearing it causes her unbearable discomfort, and she already wears the prescribed hijab and loose Islamic attire, showing nothing but her face?

The mother is required to combine covering her face with avoiding harm by loosening her head covering (khimar). If harm is unavoidable, then it is permissible for her to uncover her face, as hardship brings ease. Allah Almighty says: "And He has not placed upon you in religion any difficulty" (Quran 22:78), and His saying: "Allah intends for you ease and does not intend for you hardship" (Quran 2:185). This ruling applies unless she reaches a stage where men do not desire her nor does she desire them. If she reaches this stage, it is permissible for her to put down her head covering without adorning herself with embellishments.