Green Card For Widowed Elderly Father

My friend wants to apply for Green Card for her father who is a widower in his 80s.

He got married in early '70s and does not have a marriage certificate. His wife passed away few years back. However, marriage certificate is a necessity to apply for Green Card to prove relationship for a father. But, for a widowed mother, birth certificate that has her name is enough.

Does anyone have a personal experience in a similar situation or know someone who has successfully got Green Card for their widowed father without marriage certificate.

Thanks in advance