The surviving spouse did not need your signature or permission to file a final joint return form 1040 with their deceased spouse.
If he had an estate that had income (after his passing) that needs to be reported on a form 1041 then you can get an EIN for the estate and file that return ... it has nothing to do with the personal return. Seek local professional guidance if you are unsure of what you are responsible to handle.
Here is some general information :
https://www.irs.gov/taxtopics/tc356
https://www.irs.gov/publications/p559
