Hi Rae:  I have a similar problem with my feet and legs, but my problem is from nerve probs-sciatica from back surgeries and a injury to the sciatic nerve where my screws (from fusion surgery) broke away from the bone and went through the nerve.  I would have her checked by the Dr and see if this is related to her arthritis.  Osteo doesn't affect the nerves or circulation unless there is a fracture or something that would affect those things.  Has she been checked for any circulatory/nerve probs?  Has she tried doing simple stretching exercises that increases circulation?  If her Dr says it's ok, she could try foot flex's and making circular motions with her ankle and foot, like you are drawning an imaginary circle with your foot.  Heal lifts can help too, but I wouldn't do those unless she has excellent balance and is holding onto something (back of a chair) to balance herself.  Many different things could cause this, so I would have the Dr check it out.  Does she have any swelling in the feet?  Water retention can cause some nerve probs, but again, you would have to know for sure if she has that.

Good Luck...