I don't know why you assumed he need to move the rear sight to the right. He didn't specify what direction, left or right, he had to hold. That's kind of important.
Assuming he shot at least a 3 shot group using the center of the bull as the Point of Aim (POA), if the Point Of Impact (POI, that is, the center of the 3 shot group) is to the left of the POA, he needs to move the rear sight to the right (in the direction he wants the POI to move). Of course, if the POI is to the right of the bull, he needs to move the rear sight to the left. If the rear sight is not adjustable but the front sight is (in a dovetail, for instance), the front sight is moved the opposite way. That is, to the left if the POI is to the left of the POA or to the right if the POI is to the right of the POA.
If he is basing his statement on one round, then the above is of no help.