You should be able to use a 529 to make those payments so long as they are truly tuition payments and you are being billed in the current year. If they could be classified as a loan or some other type of fee, you might run into issues. This is because the way the IRS guidance reads, to be qualified withdrawals some of the expenses must be required by the institution and some must be incurred by students who are enrolled at least half-time. In the case of tuition, this was required by the institution, it just so happens that for you it happened in the past.
Because your situation is both unusual and unique, you might want to have a quick discussion with their financial aid office to make sure you haven't overlooked any potential stumbling blocks, but from what I'm reading you should be able to pay these bills - assuming you're being billed this calendar year - from your 529 plan.
Brian Boswell VP, Research & Development
This information does not constitute tax advice and is provided for informational purposes only. Please consult your tax advisor, financial advisor, local taxing authority, and/or plan provider or sponsor for more information.