I will try to answer what I can. Yes a person who is not a citizen can serve in the military. They have to speak English and be able to read and write it. During the 60s thousands of men from Canada and Mexico who had green cards enlisted in out military. There are more Canadian names on the wall than some small populated states. If the person in question about military service. ask them if they are still alive. Many who served in the military during both Korea and Vietnam did not go. They may have served state side or some other place. I knew a man who was from the Philiphines was in Korea. He recived a letter saying his Green Card was being taken. He was to return to his native country. He went to his CO and gave him the letter. His CO simply threw it away. When he returned state side he was locked up till the army reported he was in the service and in Korea at that time. That ended it. He retired from the Army with more than 20 years. Serving not only in Korea but also in Vietnam.