"Regular IRA. Client tells me she contributed post tax 6 years ago."

Post tax = this amount is the Basis.

"but box 2b is checked as taxable amount not determined."

Because they wouldn't know if she took it as deductible (pre-tax) or not. That's what you are explaining with how you report it, now.

"Fair market value from the 5498 from 2022 should just be ignored correct?"

Yes. It would apply for RMD purposes or if there had been a partial distribution.

"If I omit this basis"

That's not Basis. Basis = the amount she put in that has already been taxed.

"the full $5k is taxable which makes more sense?"

No. She put an amount already taxed. If this was a partial distribution, there would be a pro rata determination, such as maybe $5,000 of a $10,000 account was distributed, and her initial amount contributed post-tax would be factored, to know the % of this account that is earnings. The % earnings is the % taxable, because Basis has already been taxed.

In her case, the total distribution makes the math easy: $5,000 minus her initial contribution = amount taxable. It's the new money in that account.

Hope I got this to you in time for you to avoid her being double-taxed on her basis.