As you stated, if she had previously contributed to that IRA and had deducted those contributions on her return, then any conversion is now taxable to a %.

Example:

She put in $5,000 one year, $6,000 another year, both were deducted. And they have earnings, so let's pretend the account had a FMV of $12,000.

Now she means to backdoor for Roth IRA. She puts $6,000 as nondeductible into Trad IRA, then converts $6,000 to Roth IRA. That isn't the same $6,000. You can't pick and choose.

So, we do the math:

$6,000 Basis (her nondeducted contribution) divided by $18,000 total = 1/3 or 33% of the conversion is Basis and not taxable. That is $2,000. The other $4,000 (2/3 of the conversion) is pre-taxed money considered to be from earlier contributions and earnings and is now taxable.

And her Basis is updated: $6,000 in Basis minus $2,000 just taxed means the next conversion will use a new starting point of $4,000 remaining basis. Plus any contributions if she tries to do backdoor again.

In other words, she lost the Backdoor "nontaxable" benefit, but she still gets to put new money into Roth IRA even with high income, by using the "nondeductible and convert" tactics.