It might have been a good idea to just suggest quietly to the supervisor, that it's better to speak to parents out of earshot of others, for confidentiality?Perhaps just say that you could hear her, you were not sure about anyone else, but it would have been better for you NOT to hear, as it's not your child and it's upsetting??
BUT I think that however difficult it was, you SHOULD say something, or this woman gets away with it again and again.............:sorry: