You can find it in the "scene-by-scene" post, page 3
Next morning Luce is loading up her flower van, when Coop walks up.
Coop: "Tell me its not true girl".
Luce: "what".
Coop: "Heck doesn't know, but I figured out."
Luce just looks away.
Cooper: " you ***".
Luce : "Don't start" and walks into her shop.
Cooper follows her in and telling her how Heck called him and how busted Heck was. He mentions what Luce says about "not messing with other couples" and walks out