I think it really depends on whether or not you think your mother is capable of the after-care. My mother has cataracts in both eyes but since her AD is fairly advanced, the family decided not to go ahead. It would have been impossible for her to stay still for the local surgery, general anesthesia can cause lots of problems for older people and forget the after-care. She would have picked at any bandage or eye-patch with her dirty, unwashed-after-going-to-the-toilet, hands. Yes she would understand immediately but as you say, it's forgotten in a minute.
The consultant did say it would improve her quality of life marginally. We were not prepared to risk lots of problems for marginal improvement. Is your mother reading? Doing close work or activities? It's up to you (not the doctor) to decide as you know your mother best. In particular, investigate the effects of anesthesia on elderly people.
Good luck.