Where do you get off presuming to know all this? There’s nothing in the OP’s post that indicates either that the money was truly improperly paid to her, or that she knew she shouldn’t be receiving it.
Especially if, as seems entirely likely, it’s a matter of small or smallish overpayments over a lengthy time that’ve only just been discovered, rather than a huge lump sum. She may’ve just thought her benefit had been increased a little - it does happen sometimes, y’know.
I mean presumably if ANYONE had got a single check for this big lump sum that they weren’t expecting, then they would make inquiries about it before accepting it.
See above - if it’s a question of small overpayments that they’ve been making for some time, as it highly likely is, then she probably didn’t even notice them and it’s all been spent.
In any event it’s not like elderly folks who are on benefits are going to have a spare $70-odd grand just lying around with which to pay them back. Besides which, she may actually be entitled to it and the authorities may be mistaken - I’m sure worse has happened. I’d certainly make them double check before just handing it straight back. She could almost certainly use the money.