Anyone experienced rehabbing a sedentary elderly person back to mobility?

Here is what I suggest for someone that age who will benefit mostly from gaining flexibility vice endurance. Tai Chi is a healing art well-known among the elder in China. I suggest checking your local senior recreational center to see if there are any offering for this exercise modality. If not, then you can buy a DVD that offers a beginner level Tai Chi and she can use it at home. There are other gentle movements in yoga that most sedentary people can practice. The health benefits of both, yoga and Tai Chi are medically documented. Elderly people also enjoy gentle dancing movements. It does not matter what type, just make it fun for her.

Hey! Good job! I use resistance tubes/chords or bands with people who are terribly deconditioned. Even the lightest resistance can help build up some strength when used well, and slowly. Test it out with her, and you can do a lot with it. Just slowly see what she can and can't do, range of motion with arms, etc. and take your time with her. Even really light stretches help, and deep breathing. Again, take your time since she hasn't done anything. As for the "mercy", you may have to cut her some to start with! Being too frail can cause easy sprains, pulls and breaks!! What we see as practically nothing, many need to work towards! So don't push too hard! Build the foundation, but listen to her if she hurts. Just take time...not everyone is as strong as you are. Good job staying young and fit! Take care!

Susan-
Thanks for your reply. It codified a lot of my rambling thoughts on the issue and was very helpful.
The situation is this:
I rent a room in the house of a man who is bringing his sedentary 84 year-old mother to live here in September. He and I are both appalled at the condition she has been allowed to sink into, and I'm determined to put her back on her feet. She isn't really sick, and she isn't overweight (small mercies) but she's just terribly out of shape.
Working with her will offset part of my rent, and having her stronger will benefit everyone who cares for her and maybe give her back some kind of life.
No one is more convinced or aware than me of the critical part exercise has in maintaining health. When this lady arrives in September I'll be looking at my 79th birthday, so the poor-frail-little-old-lady excuse isn't going to cut her any mercy from me.

My grandmother is 86 and she has found that riding an exercise bike has really helped her energy level and overall health.