Traditional IRA basis after Roth conversion

If the IRA is really only used for backdoor, then there shouldn't be any confusion.

However, be aware that all traditional IRA accounts are combined as one account for tax purposes, even if they are different accounts at different brokers.  If she has a traditional IRA and broker A that has $10,000 of deductible contributions (zero basis) and she has an account at broker B that she uses for backdoor Roth IRAs which has $7000 and a $7000 basis, then for tax purposes, you must fill out form 8606 as if she had one combined IRA with $17,000 of balance and $7000 basis.

Please clarify whether your wife has any deductible balance in any IRA.

Assuming that she has no deductible IRA contributions in any IRA account anywhere, then this is what her form 8606 should look like each year (using the 2023 form as an example, the numbers will be slightly different for prior years)

Line 1 (current year non-deductible contributions) $7000

Line 2 (total basis in traditional IRA -- note this is your total prior basis from the end of the previous year) $0.

(If you are correctly doing backdoor Roth IRA contributions with no deductible balance in any IRA, your prior year end basis will always be zero.)

Line 6 (value of traditional IRAs at the end of the current year) $0.

Line 8 (total amount converted to Roth IRA) $7000.

Line 10 (non-taxable fraction) 1.000.

Line 11 (non-taxable portion of conversion) $7000.

Line 14 (traditional IRA basis as of the end of the current year--this will carry forward to the next form 8606) $0. 

If she has deductible contributions in any IRA, all bets are off the table, we would have to start looking at some real numbers.