Let's see if I can help. First, in both hitting and swinging, delayed hip action is listed (12-1 and 12-2) in the basic pattern (component 15). However, remember that in delayed hip action shoulders LEAD the backswing (with the hips leading the downswing) - whereas standard hip action has the HIPS leading both ways.
Having said that, the problem of the shaft getting underplane falls into Zone #3 -so while the pivot can pull the hands and the clubshaft too far inside and underplane (shoulder turn takeaway), extensor action is the key to maintaining the clubshaft's orientation to the plane line - hand controlled pivot!
It's sounds to me that the student in question is using a shoulder turn take away-
my opinion would be to use slow motion swings and LOOK LOOK LOOK and be certain that while the body makes it's chosen pivot motion, the hands, through employing extensor action, control the motion of the club shaft.
I will also recommend that the student make these swings all the way to the top/end and looks to be sure he is maintaining the proper shaft to the plane line orientation throughout the backstroke. Sometimes, the student turns the left wrist too much in the backstroke, and again, the shaft points beyond the plane line.
Incidently, Sam Snead was a member of my club for thirty years until he passed away, and he swung the club back in the manner you describe. BUT, at the top/end, he was able to align the components onto the turned shoulder plane for an on plane downstroke.
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