Her half of the original purchase price of the home plus half the Fair Market Value of the home on the date your father passed.
So if purchased for 200,000 her original basis world be 100,000.
If it was worth 300,000 on the date her spouse passed, her basis would be added to 1/2 that value as well, so 100,000 + 150,000 = 250,000
Her adjusted basis for the sale would be 250,000
HOWEVER if they lived in a Community Property State, such as California or Texas, she would get the stepped-up value on both halves and her basis for that sale would be 300,000.