Teach them how to trace with the right forearm. Also show them what a straight line delivery path looks and feels like. Once they understand tracing a straight plane line, they may not feel like that have to shift so much to get on plane. It will feel like they go staight back down to the ball, that should be enough to get back on plane.
Or you could experiment with them having the feel of rotaing pp#3 to the top of the shaft on the downstroke, rather than feeling behind or slightly under. Players of that caliber should pick that up relatively easy.
Threaded Mode
