I supervise a guy who, as part of his professional responsibilities, attends or hosts many public events while wearing a suit. He typically wears a single-breasted two button suit. The proper way to button one of these is to button the top button while standing, and unbutton while seated, and never button the bottom button.
He always buttons both buttons. I'm not sure if it's appropriate to say something or not. Here are the considerations that are in conflict with each other, at least currently, in my mind:
A.) It is inappropriate for a supervisor to comment on a direct's personal appearance unless the appearance impacts the direct's effectiveness with their job responsibilities. I have no idea if 'buttoning an extra button' meets this standard, or if it is just a quibble. Yet,
B.) If I'm going in front of a crowd and I have a piece of spinach in my front teeth, I want somebody to tell me. This situation kind of feels like that, and I worry I'm doing him a disservice by not saying something.
Maybe I'm overthinking this. I appreciate your input!

It’s appropriate to give feedback regarding professional behavio
Your direct's fashion choice is causing him to be dismissed by anyone who notices his professional faux pas. You can help him be more effective by giving feedback on presenting himself professionally, in this case, appropriate button etiquette. This is especially true if public presentations wearing a suit are a regular responsibility, and not all that different than giving feedback to someone for overusing "like" or "um" when public speaking.