A backdoor Roth is no different than a Conversion. It becomes "backdoor" when the person nearly immediately contributes and converts, and because the contribution (to Trad IRA first) is non-deductible, it is all Basis. As such, it converts to Roth as basis there, and there is nothing to tax (if you really have no earnings) and there is nothing to tax pro-rata (if you have no other accounts whose balances and basis would need to be taken into consideration).

I would confirm which year that contribution was tagged with.

The "backdoor" conversion is a 2023 event.

"In February of this year, she contributed this same amount to a non-deductible IRA, and plans on converting"

And that's how they end up with taxable earnings. That delay, right there.