I consider myself a Hedonist, therefore, my answer will be purely from a Hedonist prospective.

With that being said, what I personally think is that in the end, all that really matters in life is happiness and decisions should be made to maximize one's happiness or positive experiences while minimizing one's negative experience.

I can see only one valid reason for wanting to change a habit such as drinking at age 71 and that is because it is having a negative experience on one's happiness. If she is pushing her loved ones away or drinking is somehow making her miserable, then it is perfectly rational for her to want to go to rehab.

If it is for any other reason such as to improve her health or because society pressures her to then I would say that changing would be a waste of energy as it doesn't do anything to benefit her at this point.