Deductions & credits

If her husband died during 2023 (even if he died on1/01 or 12/31), she can file as Married Filing Jointly for the whole tax year of 2023.

She should indicate in the Personal Information section that she is filing a return with someone who has passed away and the date.  Please see image below.

Next year in 2024, if she has no qualifying dependent, she will file as Single.

I'm very sorry for your loss.