It's wrong to do things behind people's backs.
Kinsley and Tom are a married couple and Tom doesn't like Kinsley's new friend Tiffany, who he thinks is a bad influence on Kinsley.
Kinsley wants to go out to lunch with Tiffany.
Kinsley tells Tom that she's going to go out to lunch with Tiffany and that he's welcome to join them.
Kinsley goes out to lunch with Tiffany without telling Tom, something that she knows he will disapprove of.
Tom accepts the offer and over lunch gets to know Tiffany a bit better, beginning to form a more favorable opinion of her.
Tom finds out that Kinsley went out behind his back, and he feels like he was right in thinking that Tiffany has been negatively influencing Kinsley.