What is the ruling on an elderly widow depositing inherited money in a conventional bank and benefiting from its returns to live on, and how can this money be purified if the transaction involves usury, especially after part of it has already been spent?

The correct way to save your mother from riba (usury) is to withdraw her money from the bank and invest it in a permissible (halal) project that generates enough income to meet her needs, while supporting the project and spending on her until she becomes self-sufficient. As for her taking usurious interest, if she was aware of its prohibition, then she must donate what she took as charity, repent, and seek forgiveness. If she was ignorant, then nothing is required of her, as Allah Almighty says: "So whoever receives an admonition from his Lord and desists, he may have what has already passed, and his affair is [left to] Allah. But whoever returns [to dealing in interest] - those are the companions of the Fire; they will abide eternally therein." [Al-Baqarah:275]. However, she should frequently seek forgiveness and repent, and giving in charity what she took would be more absolving for her conscience.