I personally think that Danielle would benefit from some further training, but I don't think she will need a full three-year course, especially after two years of getting ready to be in The Wizard of Oz and being a leading lady in a major West End show. It's going to be at least two years before she could go to drama school anyway, and a lot can change in two years. I'm sure she will get classes and coaching leading up to when the show opens, and she may even get put in the the ensemble of a show like Jodie was in Les Mis. Still, after all that and whatever classes and training she gets leading up to the show, she will be the lead in The Wizard of Oz for at least a year, I would imagine, and that will be a learning experience in itself. Also, she might get some great offers for roles when she finishes Oz, so she will have that to consider as well.
She is certainly very mature for her age, and it is wise to acknowledge that she still has room to grow as a performer. Whenever the final decision is made, I'm sure she will be equipped to make the decision that is best for her. Two years (and possibly longer) is a long time, though, and a lot can happen. We will see what happens when the time comes.