I hope she knew how much income her late husband had, when she contacted you for help catching up on delinquent returns.  I hope she knew that she's liable for taxes on his income, now that she has filed joint returns, unless she later qualifies for "innocent spouse" relief.  Well, they can always open an estate and have the 2020 joint return changed to separate, within a year following its due date.  But that year may not be the problem here.  

"Know your client" is always good advice.  Especially during a pandemic, not except during a pandemic.