I wouldn't be subtle at all, only polite. The man is most probably inocently ignorant, so you should make it clear that this is very unstable, he should put the sail away right then, and should NEVER do that again.
He probably just didn't think about it. Every year some yard has a big domino, normally because of a storm, but this could easily do it. The stands are not meant for signifigant side force or for the pulsation.
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Now, if he had a catamaran, keels on the ground, I wouldn't worry about it much.
(All kidding aside, I still wouldn't do it and never have.)