Hello Karen and Virginia,
Sounds like Mom has mobility issues, a case in which people (not just seniors) are apt to think 'I can't'. There are many exercise options for those with limited mobility.
I developed a chair-based exercise program called 'Stronger Seniors'. It is practiced in hundreds of senior centers and assisted living facilities in North America. You can look it up at www.strongerseniors.com
Exercise is a habit that must be developed over time. Habits are borne out of consistency, and done at the set times. Between meals is a good time. So Mom needs to commit to exercising at 10:30 am or 2:30 pm for example. It's called making an appointment for yourself.
Start slowly, with some simple stretches for 10 minutes or so. Make up a progression chart so over the weeks Mom will work up to 30 minutes per day, three times per week. If she has a friend that will come over and participate, that would be very helpful. If you could do the exercises with her, even better.
So how to motivate (or persuade) Mom? People are motivated by fear of loss over hope of gain. Lack of exercise is one of the main causes of disease and injury. She must be convinced that if she does not start moving, chronic illness is pretty much inevitable. Loss of of health, independence, and daily function is a future no one wants to experience.
Virginia, you are absolutely correct in that exercise will lift Mom's spirits. Physical activity can prevent depression in addition to chronic illnesses.
So check out www.youtube.com/strongerseniors to see over 20 exercise video clips, and email me at email@example.com if you have any questions.