Invention's a very complex thing. Generally stuff gets credited to one person but that doesn't make it so. It takes more than one person to make something work, and even one person who has a brilliant idea usually has some kind of background or training that needs to be recognized in the account of what he or she accomplished. The inventor matters little if the thing can't be made and sold at a profit, so there's often a business partner ignored in popular versions of the invention story. And users usually change what something's for or how it's used -- what historians of technology call "the consumption junction."
How wide did it have to be? Who hammered the first box together, and who put the workouts to music? Which of those people is the inventor? If she calls herself the "inventor of step training" she's probably prevented legally from calling herself the inventor of The Step or Step Aerobics, but others such as Cooking Light might call her anything they like.
None of which is meant to downplay her role; just that the story is probably pretty complex and multi-causal. If there is a legal judgment it could form the basis for a pretty close analysis of what actually happened, but there'd still be forgotten predecessors probably: that Harvard box thing, front steps...