"All work and no play has to make Ernie a dull boy like and seriously, you in a dress? If he's otherwise busy with work, I think I might need to intervene." While she might give advice and urge people in certain directions, Lavender didn't usually meddle. Or if she did, she certainly didn't warn of it before hand. However, this was a serious situation involving formal wear and the normal rules didn't really apply.
There were few times that she needed to actually step in to do something and really, any time that formal wear was involved, it became one of those times. And it was easier to worry about Hannah's date rather than the fact that she didn't have one. "But the least you can do it let me come over and help you get ready so that he has no excuses not to spend time with you."