The way I see it, you can’t control how other people will act. But you try to do what’s right. As an adult, it’s a waste of time to get into an ego battle with a child (tempting as it might be). Your tone when talking to the parents matters. Now, if they get defensive…remind them that you’re not judging them or their child, that you only felt that it would be good for them to know.
They might have acted defensively out of embarrassment. But there are parents who’ll chew their kids out for “embarrassing” them once their in a more private setting. And if they don’t bother to do anything about the behavior…chances are they’ll hear the same complaints from others in the near future. And they might even scratch their heads and wonder, “Gee…if several people have said that about my kid, something isn’t right.”
Or the parents will eventually see the kid acting that way towards them. Regardless of how they respond…i think it’s good to let them know what has happened. Then they can’t say that “they weren’t warned” if gets worse in the future. Your intention is to help the kid and the parents…not to get the kid “busted” by mom and dad.
About an apathetic parent…hmm I don’t know how that should be handled. The more active parent shouldn’t consistently apply consequences regardless of the fact that their spouse can’t be bothered. The child should know that at least SOMEONE will bother to enforce rules.
I guess you’d talk privately to your spouse about the issue and how their apathy sends conflicting messages to the child and the lack of consistency just hinders progress and is not good for the overall home environment. And maybe tell the spouse that “Enabling our child to disrespect me is ALSO an indication that YOU don’t respect me either.”