Ok, I'm kind of bored of this, so here's a full breakdown of his technique.
red arrows are static green arrows are motion arrows are direction
Stroke preparation includes the racket coming across the shoulder.
Shoulders are pushed inwards (opposite to how you would smash).
Hips and legs are pushing outwards.
He drops the racket down to be more side on (as you would if you pronated overhead)
His non-racket side moves back slightly
His racket side moves forward slightly with his racket foot baring the weight.
He strikes the shuttle away from the body to make use of leverage, using a bevel grip.
Comments
He could substantially improve his backhand by increasing his use of his body weight. Right now he does not turn into the shot effectively, severely diminishing his hitting capability.
His contact point could be substantially improved, by hitting closer to the body but substantially more overhead he will generate more torque from the racket. By doing so his grip will actually be correct, at his current hitting position, a thumb grip would be more suitable.
His improper grip is leading to constant wide hits, an inability to hit cross, and an inability to fully utilise power from this position.
I would suggest he studies someone with a good backhand, LCW, Chen Long, Taufik, to see their contact point to improve his shot making capability.
While he may substantially believe he's found a magic ratio, he's really only found a way to iterate on the Danish swipe, and can undertake many efforts to improve. Realistically a truly reversed pronation - not quite supination, but literally pronation backwards - will generate a lot more power than his current technique.