Good info above. If the flinch is caused by losing focus on the target, a release may not help. If the flinch is caused by fear of failure, cumulative recoil, or other reasons, a release may well help.
I would suggest he train himself or get training to increase focus on the target. If barrel gets over and obscures target, an adjustment to gun hold and modification to gun movement must be made.
If, after making sure target is visible and in focus until after the shot is fired, and issue is not resolved, it may well be time for a release.
A release is not the end of the world. I have been shooting a release about 16 months, and am well satisfied with the change. I got to where the trigger finger would not move before I transitioned. Tried the middle finger, shot a 96 on the skeet field with my Model 12 Skeet. Guess what? on the last round, I began to have trouble making the gun fire.
When it is time for a release, nothing else will work.