If Box 2a says "Unknown", this means that the payer does not know the amount of the former employee's "contribution" or "basis" in the plan, that is the amount of after-tax dollars that the taxpayer contributed to the plan while working.
"She was told to subtract Box 1 from Box 9b." - this is obviously wrong unless the amount in Box 1 consisted totally of return of contribution or basis, which is very unlikely.
As the Learn More link for that line "Tax-free Amount previously recovered" says, you will need to dig up all the previous returns. The amount recovered each year should be the difference between the Box 1 amount and the actual taxable amount included in the 1040. Note that the IRS has changed the layout of the line for pension distributions several times in the last several year. Even worse, the IRS in 2018 lumped all the IRA distributions and the pension distributions into the same line, making identifying just the taxable part of one pensions distribution potentially difficult.
From what you are describing, it's possible that the previous preparer didn't account for the return of contribution (and didn't want to admit it) - you will be able to tell that when you compare the distribution amount with the taxable amount for 2018.
Another alternative which may or may not be correct would be for you to manually apply the Simplified Method calculations to the plan based on the annual contributions, the plan start date, the employee contribution, and so on.
This will tell you what should have happened each year in terms of the return of contribution. Then use the number of the return of contributions up through 2018 on that TurboTax screen. Of course, if there were one or more years in which the return of contribution was not actually accounted for on the return, then this number will be wrong, but it will give you some information that may be helpful.
For information on the Simplified Method and how to calculate it, please see IRS Tax Topic 411.
If you have the CD/download version of the TurboTax software, please look at the Form 1099-R (name of payer) - this worksheet will have the calculations of the Simplified Method on it (towards the bottom).
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