It's an unusual situation and I don't think you're going to find a definitive answer. That said, I have a very strong opinion: she cannot be your dependent because she has provided more than half her own support. This is because she is the owner of the plan.
I base this opinion on two analogies:
1. The more common question is: is the support from a 529 plan, where the parent is the owner and the child-student the beneficiary, considered as provided by the student and thus keeps her from being the parent's dependent. Here's the answer: The treatment of expenses paid with distributions from Sec. 529 plans and Coverdell ESAs in the support test is uncertain because of the dual nature of these college savings vehicles and a lack of IRS guidance. The consensus among tax experts is that it is parental support, because the parent is the owner of the plan. So, it depends on who is the owner.
2. When a UTMA custodial account is used to pay for college expenses, the student is considered as having provided her own support, as it is now her money, even though it was the parents who originally funded the account. Naming her as owner of the 529 plan is the same thing.
Have her (as owner) talk to the plan administrator about changing the owner to you.
