HI and welcome to the support group -
Dave has given you great advise, as always. I just wanted to add a couple of things~~
If your mother refuses to see her primary doc see if you have a dietitian at your grocery store. They can offer advise on a lot of different lifestyles for healthier living. I speak to the one at our grocery store on my father-n-laws diet as he is a diabetic. She has helped me learn so much about diabetes, proper sugars and how to count/divide what would be a intake for his foods/substitutions etc.,
You may be able to speak with your mom's primary doc's nurse to see if they can suggest some options for your mom. One of those would be for her to see a good spine specialist either Orthopedic or Neurosurgeon - but must be sure they are spine specialist as they are the only ones that can accurately diagnose spine problems. A X-ray/MRI/CT Scan will need to be done. I would check to see if there is a big university teaching hospital near you. I've heard some have a new MRI machine where you sit in it and it rotates around to get the pictures needed - as you say your mother is very large I don't believe she will be able to have one done in the regular machines.
Another would be for a good pain management physician - I see a PHYSIATRIST Pain Specialist - they go deeper into pain management based on patients' needs.
http://www.spineuniverse.com/treatments/what-physiatristDuring the diagnosis period, the sine specialist may refer your mom for Physical Therapy/Chiropractic treatments where they can work with her and show her proper exercising techniques.
If you can it would be beneficial for you to take her to her appointments this way you know what all is going on and also to help her with exercises at home (maybe do them with her to help encourage her).
The more you show interest in her health the more she may want to get something done about it.
Hope this helps - keep us posted what you find out~~
~~ Click on my name or picture and read my story ~~
Take care ~~ God Bless ~~
~~ Joy ~~