The dress might technically violate the rules, but I find nothing lewd or overly-suggestive about the outfit, so I probably would have let her in with a mild warning for her to be more careful in the future (if other social events are on tap).
If she had gone to the rest room, split the inseam of both legs and reattached the seams making it a dress, she would have been within the rules. That said, I would bet her ‘dressing’ was more demure and fashionable that a few of the ‘dresses’ some girls wore that night.