"Is it normal for people who have done some serious work on Broadway/regional theatre to go back to school?"
Yes I think it is, especially if they are young. She may want the extra training, she may want the degree behind her name, or she may just want the college experience.
Jodie Foster, after a successful childhood career, went to college before continuing in acting.
If anyone ever tells you that you put too much Parmesan cheese on your pasta, stop talking to them. You don't need that kind of negativity in your life.