They serve you the letter personally, don't give you a copy, and then tell you verbally what they require you to do. NSLs don't leave a paper trail on the recipient's side.
When the FBI served me with a subpoena they were waiting in the tube when I walked out of a plane. I'm sure they'd be capable of going even further to serve someone with a NSL.
It'll definitely be very easy for them to do that if she ever wants to enter the US in the future.
If your words speak for themselves, I suspect none more than "owner of keyboard." I'd be delighted to be shown otherwise.
I remain agnostic, if not healthily paranoid- which seems to me to be the general sentiment in the comments on this post.