He got a driver's license in California, let's say, and that was accepted there as adequate ID to register for SS. He might have bought a phony birth certificate, or passport. Now the Mexican Consulates and others are giving something called a Matricula Consular, which is being accepted everywhere as valid ID by the stupid.
But listen, Mexicans aren't stupid. They are just illegal and taking gross advantage of our systems which are full of holes through which Jose, Achmed, Wang, Little Johhny Malvo and his Mom are walking through with impunity.
However, in real life, Joe Rodriguez isn't going to present an SS or green card to you, he's already floated the phony docs through the temp agency who ships Joe to you in a van, pays him, and just bills you.
Now let me tell you a bad story with a good part: my daughter married a foreigner. That's the good part. Splendid guy. But when he was having trouble coping with the bureaucracy (which ain't all THAT difficult) to come here legally, he was thinking of getting a Mexican visa in his country, flying to Mexico and driving in here and letting the paper work catch up later!
I HATE THAT A FOREIGNER THINKS OUR BORDER CONTROLS ARE SO MICKEY MOUSE THAT ANYONE CAN even think about COMING HERE LIKE THAT! I assure you I would have turned his foreign hide in.