Another cause for what seems like a flinch in clay targets, particularly on the second bird in doubles (as was mentioned in the original post) is that they shooter just took their eye off of the bird (hunting for the second bird with the barrel) or head off of the gun. When the picture does not look right, the brain will not let the body react.
This may sound odd, but ask you friend to try a few shots with his middle finger on the trigger (the "bird" finger). Don't ask me why, but this sometimes works with a real flinch...uses another neural path in brain or something along those lines. Of course, if shot this way continually the new path may pick up the flinch also.
