The Air Marshals would not want ANY passenger to know they were on board. I don't think the flight crew would tell the lady, Air Marshals are all around you.
It was not the lady the flight attendant told about the air marshals it was her very worried husband that got up to go and talk to the fight attendant about the Arabs weird behavior.
It seems reasonable to me that if this was an operation to watch the behavior of the Arabs then keeping a very worried male passenger (worried for the safety for his wife and young child) calm and keeping him from interfering in any way with the investigation, would be the reason for letting him in on the fact that there were air marshals on board. They also gave him the job of writing about their suspicious behavior to keep him occupied and think he was helping.
So lets change it to, "Air Marshalls put passengers at risk in new training room in the sky".
Sorry but I really have to go now.