I have a limited understanding, but I believe the benefits for alzheimers are derived primarily from the breathing aspects of the practice.
I remember that an experiement in the 1980's showed that when alzheimers patients were put into a pressure chamber and oxygen was forced into their systems, their memory and brain function came back.
It seems to me clearly to be a problem of under-oxygenation. As I understand, in alzheimers, plaque forms that inhibits the body's ability to oxygenate the brain cells.
So, the first stage of qigong is regulating the breathing. If you can communicate this clearly to your parent: most importantly, inhalation should be very relaxed, slow and deep, completely filling the lungs. The specific movements are secondary to the breathing.
This breathing emphasis is not just for the half hour of 'qigong time'. It is all day, every day. Also, get her walking every day. As much as you can.
I think those two things will be much more beneficial than trying to 'memorize' specific qigong movements.
Also, diet is key with alzheimers:
http://www.NaturalNews.com/015553.html
http://www.NaturalNews.com/021157.html
Fast-food / unhealthy fats are linked with Alzheimers:
http://www.reuters.com/article/scienceNews/idUSTRE4AR48G20081128
Im not a doctor, but I am a geek.