When ever I see a model lock her elbow I say this: "Don't lock your elbow, but just where you start to feel it begining to lock is where I want it." This actually serves two purposes. One it prevents the model from putting a lot of pressure on the elbow and relaxes the mussels in her upper arm for a more flattering look, and two, I solve the locked elbow problem. But it does change the pose because the model can't support herself in that one spot where the elbow begins to lock and has to lean back (as Bob suggested).
In your image here just by moving her shoulder to the right 2-3" could have helped out. As long as it didn't interfere with her hair.