Air India - Allocated wrong seat prebooked and prepaid by ground staff during check in

My mother (almost a senior citizen) was to travel all alone from India to US on **-Dec on Air India flight *****L from Delhi to SFO (Non Stop). Considering her age we booked her a window seat (**A) in Premium economy well in advance and paid extra (as these seats are expensive than normal economy). But when my mother went ahead for check in she was handed over the boarding pass with seat number ** C (aisle seat) INSTEAD OF OUR PREBOOKED WINDOW SEAT which was given to someone else (not sure why). Unaware about this carelessness and unprofessionalism, my mother went ahead for boarding ONLY TO FACE ARGUMENTS FROM FLIGHT STAFF AND DIFFICULTY IN SITTING during the entire ** hours journey. Just to mention, she is a sleep apnea patient and has also to carry her medication and machine all the time with her. She tried to convince the flight attendants BUT THEY HARDLY PAID ANY ATTENTION to her. This was the worst flight experiences my mother had to have (Even after having everything planned and paid well in advance).