The 'Good Charlotte' rocker and actress Diaz sparked rumours of a new romance when they were spotted spending time together in May, reported Us magazine.
Now, Richie, who is married to Benji's twin brother Joel, has opened up about her matchmaking project.
"I approve of anything that's going to make Benji happy. I am a devoted sister-in-law. I'm happy for anyone who is happy, and I want everyone to be surrounded with love," the fashion designer said.
When questioned if she was the one who officially set Madden and Diaz up, Richie admitted, "Yes. I'm going to take responsibility for everything!"