I went to the Marine Corps Basic School (all newly commissioned Marine Officers go to the Basic School in Quantico Va) in 1981 and we had a Canadian in my company. So maybe the rules have changed in the years since.
There are still Canadian Marine Officers. They might have trouble getting certain MOS’s due to clearance issues, but they are there.
Yes, and I had all sorts of foreigners and other services in my basic courses, advanced and service schools throughout the ranks, but , as you mention “all newly commissioned officers” attend the USMC Basic School. I think he was an exchange officer, no?
Been out of touch a bit, so sorry for the long delay from the OP.