There are two ways:
1. Show nothing at all. If all the 529 plan was used for qualified expenses, nothing goes on your tax return to show this.
2. The 529 plan will issue a form 1099-Q for the distribution. There is a place to enter it in TurboTax (TT). One of the choices will be "a beneficiary who is not a dependent on my return". But when you get all thru the complicated interview, nothing goes on your tax return. TT will give you a work sheet for your own records.
This works if your ex does not claim anything, for educational expenses, other than what he actually paid.
For a more detailed discussion, see
https://ttlc.intuit.com/community/college-education/discussion/college-expenses-my-federal-tax-incre...
