I took a flexibility yoga series from Michigan State University that was strictly for OA and fibros. It, along with hot showers, got me feeling better. The instructor who taught it had severe arthritis in his spine from a bad car accident over 20 years ago. The yogic theory is that the movement of the joints increases some of the synovial fluid production in the joints and helps with mobility and pain. It was uncomfortable in the beginning of the series but I started feeling better within two weeks. You could do this with her a couple of times a week and have her take a darvocet about
20 minutes before starting. She may also start out in a chair if she can't get to the floor.
The flexibility series is not a bunch of weird poses or strange postures. It's simply moving things to help them stay flexible. When starting the series instead of 5 reps, try three, then the next time four until she can work up to 5. It's free, it helps reduce stress and pain and there are no side effects. Plus, she gets to show off to her friends and tell them she is doing yoga.
This is the website with instructions.
Flexibility Series