I believe even with a high tolerance, our bodies can only take so much abuse. The combination she's on is a recipe for respiratory depression. Most just go to sleep and never wake up.
I can understand her throwing up excuses like back pain, etc. But the bottom line is she's not taking the meds the way their prescribed.
Snorting and chewing?
I'm not sure how to get through to a friend that far in. The painkillers are one thing, Xanax is a totally different beast.
Off the top of my head I'd ask to go with her to her next appt and observe what's going on in that office. Are they on the up and up? Is she getting meds on the side/illegally?
Is there someone you can talk to for support? Is there other people in her life? Kids/partner/family?
Last - does she have any desire whatsoever to take less meds? Or stop completely?
I'm sorry this is happening. It's really hard to watch anyone go through this. (((((hug))))