The Verdict: Clear Path
She can reacquire Greek nationality by declaration under article 22 of the Citizenship Code, with no residence, test or deadline. She should make the declaration at the Greek consulate or a Regional Directorate of Nationality.
Before the 1984 reforms a Greek woman could lose her nationality by marrying a foreigner. Article 22 reverses it as a right, not a discretionary grant, and article 24 has it confirmed by decision.
She needs proof that she was Greek, such as her municipal register entry, the marriage certificate, and any record of the loss.
Only she can make the declaration. If she has died, this route closes with her, and her children's position is a descent question: our Citizenship by Descent quiz covers it.