We dont do anything like this either - playing one parent off the other is a punishable offense. I did this as a kid between my mother and Grandmother (whom we lived with) and it wasn't good. At times I'd even threaten my grandmother with telling my mom that she'd allowed me to do something if I wanted to do something else. I do NOT want my kids doing it!
That being said, I would have said something to "Adele" at the time about not keeping secrets from her mother and moved on. Even as strict as I am about it, I would not expect to be told. Just assume the kid is kidding or whatever and let it go.