Yes, he can.
But he will be violating US federal law if he does.
Check your facts. We can export up to $500 of gun parts..no permits or export doc or us exporter.
Got a lot of gun parts, handgun barrel with Compensator, mags, sear, triggers ship by mail and through CBSA and absolutly no problems, and this from numerous sources.
For company not shipping in Canada, I use a freight forwarder and pick up the goods at the border. ( living 1.5 hours away). Fully legal.
Just ordered 10 mags from Brownells..and the products they sell are not from a licensed manufacturer exporter.
So you can wave the rule book as much is you want, this is how it work now, and it’s fully legal, both side of the border.
If it’s parts for guns sold in Canada to consumers, most likely there will be no restrictions and the $500 apply. For over $500, export permit is required. Of course this does not apply for a machine guns parts, suppressors and the like.
But for the OP hunting mag..go ahead, just get it.