I know it's bad etiquette to invite someone for just the dancing portion of the reception. However one of my friends that I invited just let me know that she couldn't make to the wedding after all, because it's her boyfriend's birthday and they have plans for the day. But she wanted to know if it was alright for them to come after dinner and hang out and enjoy the rest of the evening with us. Is this against etiquette, even though THEY are the ones asking to come just for the dancing?