Is it obligatory for a grandfather to financially support his deceased son's daughter, knowing that her father left her a monthly income and her mother is employed?

The aforementioned granddaughter is not entitled to financial support (nafaqa) as long as she is not in need. If the grandfather spends on her voluntarily, seeking the countenance of Allah, he will be rewarded for upholding kinship ties and for every expenditure made purely for the sake of Allah the Almighty.