Since she already made a TIRA contribution for the full amount, she has used up her full regular IRA contribution limit and cannot make a regular Roth contribution or any TIRA contribution as well.
She should also report her TIRA contribution as non deductible on Form 8606 (same form to report the total conversions). As such she will have 6000 of IRA basis and total conversions of 21,000 will have a taxable portion of 15,000. At a marginal rate of 24% (plus state rate if any), her conversion taxes would be 3,600 plus any state income taxes due.
If she does not do an additional conversion above her 6000 conversion, the taxable portion of the 6000 conversion will be 4,286.
Now, if she were not to convert the 15000 this year, AND if her MAGI for TIRA deduction purposes was low enough for a full 6000 deduction, she would reduce taxes somewhat this year by claiming the TIRA deduction of 6000. That would increase the 6000 conversion to 6000 taxable instead of 4286, but the 6000 deduction would reduce the net taxable amount on her return by 4286 in total.