As has been stated - tell them what you saw, tell them what you believe/know the rule to be and have your say. Then, be done with it and move on.
What's that old expression, "Never argue with an idiot. He'll bring you down to his level and beat you with experience."
I'm not saying they're idiots, but I find some vets/trainers "don't want to listen or g_d forbid, admit they're
wrong even if you show em in the book. Shrug.
Have your say, know you're right, and move on.
PS - took me a while to learn to shut up.