Hire the expert
The property owner needs to trust your expertise.
Power brooming removes the crap from the surface of the grass.
Called brooming, it is done with rubber paddles.
Using bristles is fine on hard pavement surfaces, but would damage the grass.
I work at about 20 minutes for the first 500 sq ft. It is actually work because
I have to push the unit forward while it pushes backwards to clean. I follow with a quick blow to fluff up the grass.
The property owner is suggesting Power brooming which removes the crap from the surface of the grass. If you, the expect, think that the lawn needs the dead debris needs to be removed from below the grass surface tell him what he NEEDS done and HOW it should be done. If he does not want to to it properly tell him he will see poor results from not following his Lawn Expert. Be nice, Be professional, Be firm.