Leslie Sansone has a DVD for older people who are just getting started called Walk For Seniors Aerobics.
http://www.inspiredcorp.com/leslie-sansone.htm
If you do a search for senior exercise dvd you can probably find lots.
If she has no history of exercise she should start slow--walking is probably good. Make sure she has good shoes, not just some little flat slip on things.
I'm sorry to hear your mom has Alzheimer's at 65. That's pretty young. I assume a doctor has diagnosed her.
Good luck,
Jay