Ok the Law fo Specificity.
The Law of Specificty dictates that to get good at something, you need to do it.
* Weightlifters perform the clean & jerk, to get better at the clean & jerk
* Powerlifters perform the bench/deadlift/squat to get better at these exercises
* sprinters sprint
* swimmers swim
etc.
But because of the Law of Accomodation, you see weighlifters, powerlifters, runners, swimmers perform different but similar movements.
Weightlifters will do snatch pulls, heavy back squats,...
Powerlifters use different bars, lockouts, board presses,..
Sprinters do uphil sprints
etc...
Not wanting to believe that you get better in pullups by doing pullups is your right. Not believing in the law of specifity/law of accomodation is neglecting nature's laws.
Let him do pullups daily for 5 sets of 5. After one week he can report back his result. They'll be positive.