Q. Who is the party potentially subject to the tax - the account owner or the beneficiary?
A. In this case, the beneficiary*.
For 529 plans, there is an “owner” (usually the parent), and a “beneficiary” (usually the student dependent). The "recipient" of the distribution can be either the owner or the beneficiary depending on who the money was sent to. When the money goes directly from the Qualified Tuition Plan (QTP) to the beneficiary's Roth IRA, the beneficiary is the recipient. The distribution will be reported on IRS form 1099-Q.
The 1099-Q gets reported on the recipient's return, if it even needs to be reported. The recipient's name & SS# will be on the 1099-Q.
*It actually does not get reported. So far (for 2024), the IRS has not come up with a way to report it like (for example) how you report an IRA rollover. The IRS was proposing a new check box 4b on the 1099-Q for a 529 to Roth IRA rollover (trustee to trustee). But, I have not heard an update on that.
