For me, the grass is the problem.
I think I might start by telling him that performing a roll cast well on grass is bloody tricky.
I might then go on to say he would be better attempting this cast on water first in order to grasp the essentials of making the cast - but mainly getting his head around tension. The lack of which is the enemy of the grass roll-caster.
If he is expending
more effort in achieving the roll cast then, to me, he is putting too much effort into the cast and that has to be down to a lack of tension which in turn means he's not loading the rod properly and not making an effective cast.
It might boost his confidence to make the casts successfully on water
first then see if he can adapt to grass subsequently
Will the assessment be done on water or grass
If on water then it's a no brainer (to me) about where he should practice and continuing to practice on grass will almost certianly lead to bad habits.
H