If AA sues, it sues based on a breach of contract theory. The damages are the difference between the fare agreed to and the amount paid. That is a simple factual determination based on arithmetic. Not likely to be a dispute.

Whether AA makes or loses money on any given ticket is neither relevant nor even applicable.

If the passenger sues, she must find a legal basis to do so. A little bit hard to overcome when one has agreed to a contract which expressly prohibits the conduct you engaged in.

But, AA is savvy enough to know that it need not sue anybody for anything. As is commonly the case in these matters, the customer does not walk, but wants to keep flying whatever carrier it is AA knows that too. Thus, a perfectly polite and thorough note and it has ended the practice for at least one customer and likely made back the costs of its investigation.