This thread might help:
Yes, other people's success stories will show her it can be done, but is there some activity she's keen on? Finding something she wants to do and working towards that will give her internal motivation. I think a lot of older people tend to assume once they've had hip replacement etc that they have to slow down and be careful. Does she like dancing or walking (as in hiking country trails)? Either of those can be done as gentle slow exercise that will improve mobility and help overall health. There's very little out of her reach (other than Olympic competition) as far as activities go.
FWIW, my father had a hip replacement at around 75 when he couldn't walk more than about 100yds. But his favourite activity at that time was walking, especially in European mountains (Pyrenees, Dolomites) so once it was replaced he was up and walking around and spent another 13 years caravanning and walking up mountains before failed eyesight stopped him driving.
Me, My Father and The Alzheimer's - http://www.marksdailyapple.com/forum/thread84213.html