If it's genuinely just savings, it might raise flags but not lead to further actions if she can explain that she's just lived with cash up to now, it's been over a few years, and wasn't benefit fraud.
How did she come by the cash? Did she either receive a pension or other income in cash, and she's just saved some by not spending as much as she received? Or it was paid into a bank account and she can show regular cash withdrawals?
She could try to pay it in by a few instalments, but it still might raise flags if the system spots her doing this, but she could come up with reasons if they ask in the bank like 'I sold a car'.
But then again, it's getting harder to get larger amounts of money out of banks these days, because people don't need them for legitimate reasons, so banks don't make it easy and could be suspicious about any large amounts being paid in.
Even people doing things like buying second hand cars from private sellers can send the money instantly using their phones so don't need to use cash. A friend tried to get a few thousand pounds out of his bank account to pay a builder under the table and he was turned away because they correctly suspected it was aiding tax evasion.