Sounds like you've covered most of it.
I guess the problem is that he is taking too much weight on his hands and not letting the harness take the weight?
Either that or he is rotating the body and leaving the hands behind? Both can lead to back hand pressure and a kite heading into an up-loop.
Maybe get him to do a few basic jumps where he really concentrates on keeping a light touch on the bar and letting the kite lift him via the harness, then have him try and re-create that feeling during the backroll.
Also, tucking the legs up helps speed up the spin and get around quick before the kite gets too out of control.
The soft option could be to rig a rope from a tree and attach himself with his harness and a mock-up bar and practice spinning underneath it (while humming the song "see-saw, marjory daw..." etc).
The hard option is of course to forget the basic backroll, just get both hands on the back of the bar and commit to the F-16 while screaming some death metal lyrics !!

...your friend needs to make that call!