I never think this is a good idea. Most people are in denial when they have bad ass kids and that’s why they get defensive. I only have one rule about it and that is, if you are in my house you play by my rules. If I am not allowed to discipline your child while they are in my care then you can’t come over. Otherwise, I would just keep the interaction to a minimum and mind your business. It’s their cross to bear, not yours.