To some, Christian crossed it at the climax when he attacks and shames Satine in front of an entire audience. On the other hand, this is actually part of the play (you see Christian writing the scene earlier on), with Christian standing in for the unconscious Argentinean, though undoubtedly the theatre troupe is aware of the double meaning.
Took this out from under Moral Event Horizon because you can't really say The Hero crosses it. However, perhaps it could be re-written and added under Alternative Character Interpretation: