Bump it up to a 20 gauge if possible. Killing a turkey can be done with a .410, but far more times you will have a miss or a wounded bird. If he can shoot a .410, he should be able to shoot a 20 gauge. I personally wouldn't even consider sending someone out turkey hunting with a .410, just my opinion. If you are determined to use it, then I would go no further than 20 yards.