I don't think that they crossed a line legally (although maybe they did, I don't want to count it out), but they definitely crossed the line morally.

Depending on how person A feels about being revealed (clearly, that's very different depending on whether person A is publically active, a minor celebrity etc etc.) I would in their place either complain to the airline, or ask the mutual friend to lay it on thick to the flight attendant friend that this is not OK and if they do this to a wrong person they will lose their job.