"最坏的结果不过是被扣在那里,让我给她寄文件" - we wouldn't know w

In my opinion, the LUCKIEST outcome is what you called The Worst outcome.

If her visa is truly invalid, and TSA goes by the book, she may be sent back and be barred from re-entry. Also she now may have a notation in her file and the long term effect is anyone's guess.

Best of luck to your friend.