Who reveals the truth about Satine and Christian to the Duke in the movie Moulin Rouge?

In the movie "Moulin Rouge," the truth about Satine and Christian's relationship is revealed to the Duke by Zidler, the owner of the Moulin Rouge. Zidler does this in an effort to protect Satine and the theater, although it ultimately leads to dramatic consequences for all involved. His revelation serves to heighten the tension and conflict between the characters as the story unfolds.