As always, your mom should check with a doctor, but I do believe the upper body pawanmuktasanas would be really helpful for her. You can tell her to go to the web site and read those pages ... here is the link.
http://www.yinyoga.com/ys2_2.2.7.1.4.2_ ... tasana.php
Moving the fingers in this exercise helps to free up "chi" or prana as well as simply moving the blood and synovial fluids.
As to compressing the knuckles, I do believe that is important. Physiotherapists would probably do that for her, and physio is not without a lot of discomfort. But to be sure, maybe she should see a physio?
I would be curious to hear what Mary (Massung) has to say on this topic...Mary?
cheers
Bernie