After being asked by a fan if she approves of their new romance, she said "I approve of anything that's going to make Benj happy."
Richie, 32, married Madden's brother Joel in 2010 and they have two children together.
"I am a devoted sister-in-law and I am happy for anybody who's happy," she added. "I want everyone to be surrounded with love."
Richie added that "yes," she set them up and "I'm going to take responsibility for everything."
Pictures of Madden and Diaz surfaced early last month of the two holding hands, and more recently, Madden has said he considers himself "lucky" when it comes to his love life at the moment.