Chances are that there is a defective A.B.S.(Antilock Brake system) sensor. This is allowing the brake pedal to pulse when he applies the brakes.
Yours and his best bet is to take the truck to a reputable brake shop, one you trust, and have the A.B.S system checked.
Before you have any work done or sign off on a repair order, check here to see if the diagnosis is legit.