It seems that she has deep cavity there. If she has pain associated, she probably will need root canal treatment with that tooth. She will probably need crown/ cap on that tooth after root canal treatment.

Hi..based on your mother's complaint and picture, the tooth requires root canal treatment. Visit nearest dentist to initiate the treatment. Regards

Hi good that u send pics..as it appears in the pics she might have an exposed root canal of the tooth involved ..the cavity seems deep ..she might need a root canal treatment for this..visit ur dentist for best advice take care best of luck

Hi, what she is suffering from is a decay which has involved the pulp of the tooth ( the nerve) that's why she is having pain and is difficult to bite...an x Ray will make things more clear but what I see clinically and the history as given by you....she will need a Root Canal Treatment for the same....and that should relieve her from this pain. Regards, Dr. Vishal

First of all this condition is known as cervical abrasion and it is due to bridk brushing so first tell your mother to use soft bristles toothbrush secondly it has involved the pulp of the tooth so better get it treated