Your relative should consult with the NSA -- with an office of hiring, human resources, or equal opportunity if you can find contact information for them -- about agency-specific protocols.  Sometimes, when you can't find the right phone number, an office of public affairs or field office contact can help. 

In general, a FOIA request of this type should look something like this: 

Dear Appeals Officer/Other Governmental Agency Contact,

Please find attached a notarized statement regarding my place of birth and citizenship status. 

Please consider this letter my investigative file [or other term the agency uses] request. 

Sincerely,
Cousin Freestate


The less specific you are, the more likely your request will meet with a substantive response (because you risk inadvertently ruling out information when you make it more specific).  So keep it simple. 

To answer your other question, I think usually when people on these boards say LE, it's shorthand for law enforcement. 

Good luck.