"My daughter has been dancing all her life without a break. Ballroom is her final frontier and recently took her first private. It was only a 50 min. class and it left her sore. Huh....ballroom dancing leaving my daughter sore?"
Different physical tasks require different physical strengths.
Another issue to be aware of is that doing something the wrong way can be much harder on the body than doing it the right way. You didn't say where she feels sore, but one example comes to mind which needs special caution: Many less experienced lady dancers carry a large amount of arch in their lower back in an attempt to imitate what they think the professionals are doing to create a big shape. The situation is not helped by inexperienced male teachers who encourage this to get quick "results", or by inexperienced professional ladies who do it themselves, but it is neither proper technique nor healthy in the long run, and pain is one of the first indications that something is wrong. Proper shape has the curve distributed over the entire length of the body, and while challenging is not nearly as taxing as doing it the wrong way.