Maybe there's something I'm not thinking of, but I don't see how Dwight is the birdie yet...he just doesn't know enough I dont think. At this point in the story, all Dwight really knows is that Rosita was at the Sanctuary walls with guns. Maybe he saw Sasha go in too, but if he were talking with Negan then he would know by now that she's been captured. But seeing two girls at the fence isn't enough IMO to tell him that Rick has a plan. If an enemy was going to coordinate a plan that involved sending two "scouts" or infiltrators into the Sanctuary, they wouldn't send two girls. Not being sexist here, just seriously doubt that Dwight or Negan would see Sasha and Rosita's attempt as a coordinated move by Rick. Rick wouldn't send two girls. Nobody would (except Oceanside, since that's all they have). Rick might send a girl, or two girls, but he's going to send a dude with them as backup.
I seriously doubt Rick or anyone else is going to share any information at all with Dwight about their plan now that he's at Alexandria, not until they figure out a way for Dwight to prove that he's really against Negan somehow.