"for her to be eligible for 2019 you need to drop your coverage of her before 12/1/2020"
I think that this is a typo, s/b "for her to be eligible for 2019 you need to drop your coverage of her before 12/1/2019" or "for her to be eligible for 2020 you need to drop your coverage of her before 12/1/2020"
Do I understand you @pgdiehl to say that your daughter will turn 26 sometime in November of 2020? If so, the following things need to happen to get the ability to contribute to an HSA for tax year 2020 (as opposed to just IN calendar year 2020, because you can still contribute to your HSA for tax year 2019 up through the due date of the return in 2020):
1. She must be covered by an HDHP no later than December 1, 2020.
2. She must NOT be covered by your FSA in any capacity on December 1, 2020; i.e., there is no way that she can file a claim for FSA reimbursement on or after that date under the FSA plan rules (this is a question for the FSA plan administrator).
3. She cannot be claimed as a dependent in tax year 2020 (i.e., next year) - note that it does not matter if she actually is - even if you don't claim her as a dependent but she otherwise satisfies the requirements, then she cannot contribute to an HSA.
4. Because of the last-month rule, on or after December 1, 2020 (up until the due date of her return in 2021), she can contribute up to the annual HSA contribution limit for 2020 (which is normally updated by the IRS each year).
5. If she does this, however, she will be required to stay under the HDHP coverage (and otherwise qualified to contribute to an HSA) through all of 2021 ("the testing period"). If she changes jobs or the employer switches to a non-HDHP insurance, then she will have to reconcile this (pay tax on the tax benefit she got from using the last-month rule) in 2021. However, don't worry about this until it actually happens and she files her 2021 return. If it happens, TurboTax will walk you through it.
