People are missing the point of this sort of service. It is not a free for all chat - you have to setup who you want to chat with and they have to accept.
The biggest use is for families and groups that are split up in their seat assignment. When you're trapped in a middle seat miles away from the rest of your family it's quite nice being able to send messages during an 8 hour flight without having disturb multiple people by going to chat in person each time.
I'm quite sure the cabin crew can deactivate it or block seats. She could have turned chat off herself instantly too - you only have to flip back to the TV channel to no longer see it.