As others have said, details of the original marriage would be a starting point to helping.
Without meaning to sound rude in any way, you must remember that people don't always tell the truth on official forms.
For example, other possibilities include your father may have been divorced or simply split up with his first wife rather than actually being a widower.
In which case the first wife may have gone on to remarry and her death would have been recorded as a different surname.
To give a specific example of people not telling the truth, I have a female relative who gave birth to two daughters and on their birth certificates gave the name of their father as her deceased husband who had died several years before they were born.
Or another relative who, on one census, described herself as a widow but in reality she had separated from her husband at the time and they later got back together and had more children together.
Or another relative, one of my mum's aunts. She married a guy and they had one daughter together. She then left him and had two children with some other unknown guy or guys. Then she later shacked up with another guy and took his last name and claimed to be married even though they never did marry.
There was no record of a divorce and her original husband later remarried 33 years after my mum's aunt left him. He also described himself as a widower.