I also think that she could have very well told Monroe everything, although I have heard good arguments to suggest the contrary. Still, I think Monroe would have guessed the truth in the long run.
But back to the topic, Ennis must have known all along, given the way he so thoughtlessly behaved -- hardly concealing the truth behind the fishing trips with Jack. He simply assumed that Alma would not bring it all out -- in this, he was mistaken.