maybe by "green card" they mean a green light didn't light up when he scanned his access card. <badum-ching>
Datacenters are sensitive facilities. It's not surprising that they block entrance to many people.
From what I've seen in Europe, I can remember a couple of places in military, datacenters and national research centers where it's clearly written "European Only" on the jobs.
"A Green Card (Permanent Resident Card): Gives you official immigration status in the United States. Entitles you to certain rights and responsibilities. Is required if you wish to naturalize as a U.S. Citizen"