Stupid part of it is that there is nothing stopping him from buying it once he got here. Body armor is not regulated in most places in the US.
“Stupid part of it is that there is nothing stopping him from buying it once he got here.”
In practical terms you are correct, but legally if he’s a foreign national on a visa not a Green Card, ITAR still applies. He could not, for instance, purchase ITAR restricted RF chips or Krytron switches.
If he found a dealer who did not check for citizenship then yes, he can buy ballistic armor, but the transaction would not be legal.