Stay out of it if at all possible. She's a grown adult, and she's making the choice to quit, so paying for things she wants/needs is all on her if she won't save/reduce spending or budget.
If she has SS of that much a month, she technically should be fine. If she can't afford her apartment, she'll have to get a job again or move to a cheaper place. Do not subsidize her. She can't move in with you. And you are not going to pay her bills so she can spend on stupid shit instead of the basics like food/utilities/shelter if she chose to blow through money and not build up any savings to support herself and insists on quitting working besides.
Where does she think the extra money is going to come from to pay for things like food, electricity, gas, insurance? If she knows the SS amount and knows her apartment alone is more than the SS payment, there's a disconnect, so you can begin by asking her how she's planning on paying for everything on SS if it doesn't cover her current expenses.
Tell her what you can and can't do for her: sit down and figure out a cheaper place to live, maybe help her find a roommate or room to rent with utilities paid situation (I see those often on Nextdoor for half the cost of renting an apartment/house and utilities are usually included). She may not be able to quit her job, and you'll need to explain to her that if she comes up short on something to the point where she gets behind on bills, you won't be paying off her credit cards or letting her crash in your spare room or loaning/gifting any significant (or any) amount of money; you'll provide help researching, advise her on money/bills/debts, help her fill out paperwork or make phone calls and be a sympathetic ear, but you are not responsible for her life choices, and she's going to have to do the work or cut expenses if she wants to retire and stay retired.
You need to make sure she understands the amount and type of help you are and are not willing to give her.
If she was doing okay, still working and accidentally got behind due to illness or something else beyond her control I could see helping out, but this isn't that type of situation. She's determined to quit her job? She should have saved up enough to make up the difference to pay for herself. Period, full stop. This isn't your responsibility to make up the difference, and you can tell her that you don't have the money in your budget, so she'll likely need to get a job again to support herself if she didn't plan accordingly.