new passport -marriage certificate not available, husband expired.

The query is on behalf of my mother, my father is no more and my parents do not have a written proof of their marriage (as it is required as one of the proof to apply for passport since my mother surname changed after marriage), what my mother do possess are the following set of documents-

all her school board certificates,
her graduation certificates,
ration card (bears her name and mine, as it is taken recently),
PAN card,
bank passbook,
legal heir certificate,
driving license,
house and property documents(on my mother's name),

all the proofs have been obtained after my father's demise and thus the husbands name bears the "late" tag on some of those proofs.
Kindly describe the process to be followed to apply for the passport in the above case.

Self declaration would be enough in this case. She will not be requiring marriage certificate.