This is a common Catch-22 problem. It hurts to walk so the person stops walking. But walking might help reduce the pain. In fact, studies show that walking will very likely decrease her pain. You might start by sharing that information with her.
Sometimes it helps to point out that the person has pain whether or not she walks, so she might as well try walking. It might help! And it has other health benefits besides improving back pain.
Sometimes the person just needs someone to get them started. Encourage her to just walk around her house once each hour. Or suggest she take a 10-minute walk. If she lives in an apartment, suggest walking down the hallway and back. If she lives in a neighborhood, she might try walking down the street and back. Start with once a day and increase it to two or three times each day.
If you can’t walk with her yourself, try to find a friend or neighbor who can walk regularly. Don’t force or push her to walk faster or farther at first. Just get some walking started, and see if that doesn’t take care of the rest.
If none of your efforts work, perhaps it’s time to talk to the doctor about pain control first. A short course of physical therapy might help break the pain-spasm cycle. Once the pain is under control, the therapist can supervise her in a walking program to get her started.