First, I'm sorry for your loss.
When you file their 2019 return, you will want to be sure to say that he is deceased, even though he didn't pass until 2020.
For their 2020 return, they can still file jointly regardless of if he had income or not.
The Portability you are referring to is an Estate Tax issue and regarding gift tax.
In order to elect portability of the decedent's unused exclusion amount (deceased spousal unused exclusion (DSUE) amount) for the benefit of the surviving spouse, the estate's representative must file an estate tax return (Form 706) and the return must be filed timely.
This IRS may provide you additional insight: https://www.irs.gov/businesses/small-businesses-self-employed/frequently-asked-questions-on-estate-t...
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