I have nothing of any medical or therapeutic value to offer to here, but I've been following this thread with interest and something came to mind after your most recent post. From
this article:
Your friend is afraid to perform an exercise that will strengthen his back in a position of rigid, anatomically normal extension because it involves added weight, but is perfectly happy to wiggle his already damaged spine through flexion and extension because it doesn't involve added weight. I get that you don't want to discourage him from training, but I think he needs as much help getting his mind right as his back right. Have you had him do any reading around here, or are you just hoping he'll take your word for everything? Referring him to some of the many articles about back pain and back strength might be helpful, if you haven't done so already.