A group "Thank You" to the *many* who responded to my question.  I can't begin to answer you all personally!
By researching on my own, I learned that the hormone is called "relaxin", and that it is (apparently exclusively) a product of the corpus luteum.  That being the case, there is simply no way that it is related to lactation.  I explained this to mom, and suggested that she ask for references supporting the contention that lactation is implicated.
Several Lactnetters sent me their personal stories of similar joint-pain problems, and I even dimly recall that I suffered from "sciatica" on and off for years during my childbearing/lactating years.  I suppose I never really registered that it went away...
Thanks Again.
Willow