I see this all the time, and, as you noticed, it typically involves an elbow flaring out as the lifter drives the bar up off the chest. You have to keep cueing her to tuck her elbows. Start with a light weight she can do perfectly and work up from there. Make sure she's bringing the bar down low enough on the chest/sternum. She has to learn how it feels to drive up with the elbows a little closer to the ribs. Demand perfection. That's the only approach I've found that works. Perhaps some of the other coaches will weigh in with their tips.