You should always dress appropriate for the occasion in my opinion. If that means a rental or a conversion, then sobeit. However, one should never criticise another regardless of their dress. One must always assume that circumstances, whatever thye might be, prohibited the individual from getting the "appropriate attire" and accept them with grace and courtesy. Good manners is not only dressing appropriately but conducting oneself appropriately for the occasion. I have been to many formal functions once, because someone appropriately attired acted like a boor.