I suppose I could be said to have "tattled" on a child - I went to the store to shop today, and while I was there, I saw a girl playing with a display piece which she had apparently played with to the point she had taken it apart. (The piece was a shelf-front display and had a small button on either end; the buttons clicked into holes on the shelf build-up. While I was watching, it was evident that one of the buttons had come free of its hole, so the display piece did not sit properly.) It is possible that some other child had taken the piece apart. When the male adult who was shopping with the child started to walk away, having not observed the child's behavior the entire time I was present, I advised him that the girl he was with had been playing with the display and had apparently taken it apart. He had the grace to put the display together, at least.
I wasn't about to let him just walk off completely oblivious to his child's behavior, and the effect of what may have been his child's play. While I did not see *her* take the piece apart, I had no intention of fixing the results of another person's play - and especially not that of a child whose parent had permitted her to treat the display as an interactive toy.
I think this is different than what Shannon's Mom is doing. I'm a big advocate of providing other parents with "undercover intelligence"--information about their child that they're not in a position to know about but I think they might want to know.
But I'm certain that I wouldn't create the reaction that Shannon's Mom does. Partly because of the tone I've chosen the few times I've had to do it--it's not, "she did something wrong!" but "I know of something, and I'm being respectful to your authority in providing you with information, and what you do about it is NOT my business."
I agree w/ bopper: Act as though your ODD did nothing wrong.
And I like "why are you telling me this?"
Or, "I don't see any reason to get involved."
or maybe, "I'd prefer only hear about the big-picture stuff. I'm sure the kids will figure out how the dynamics of their friendship works."
You've ALREADY DONE the other thing I'd suggest, which is some independent research seeking out neutral opinions about your DD's relationship
with Shannon, and whether she's sort of mean to you. (that's the "big picture stuff" I'd want to know about)