[MD Longhorn]

Try it from a different tack then. I understand where you wouldn't want them to get flogged publicly...

Next time you're working with them, have the book handy. Start with, "Remember that illegal shift we were talking about last week. After we talked, I dug around in the book to refresh my memory on the rules, because obviously I was missing something. Here's what I found (read them the relevant rule). What am I missing? I can't find the rule that says once one of the two players stops, it's legal motion."

That might not be perfect, but it A) shows them you are interested in learning, and B) shows them they were wrong without you having to be accusatory. Best case, they'll say they misunderstood the question the week before, and you're right (and they'll start calling it correctly in the future). Worst case, they'll be upset, but will start calling it correctly.

Even veterans make mistakes - but there's no good reason to allow the mistakes to continue. Everyone should learn from mistakes.