Under U.S.code, foreign nationals who have been lawfully admitted to the U.S. can purchase and possess a gun under a few specific conditions:
If they were admitted to the U.S. for lawful hunting or sporting purposes, or if they are in possession of a U.S.-issued hunting license or permit
If they are an official representative of a foreign government who is accredited with the U.S. government; if they are accredited with a government-partnered international organization headquartered in the U.S.; or if they are in transit to or from a country where they are accredited
If they are an official of a foreign government or a distinguished foreign visitor as designated by the U.S. Department of State
If they are a foreign law enforcement officer of a friendly foreign government entering the U.S. on official law enforcement business
Looking at the history of people at the Department of State... I'm gonna say there is no way the Department of State should have the authority to allow foreign visitors to be armed or purchase firearms.