Tell her that she wouldn't go to the bank each morning and afternoon just to use her keys to get in and out of her own home, because she trusts the bank more to keep her keys secure than her own pockets.
And then tell her to imagine the banks have ZERO accountability, and at any time they can close shop and run away with the customers house keys, and you can't change the locks to your home because it's literally impossible.
And then ask her why she's doing this with her crypto?
Now that said, I would still say that for computer illiterates, it's much safer to offshore security to third parties, because at least they'll try protect her from herself. But if she knows enough to be into crypto, she should go the extra mile and learn about security.
If all else fails, tell her to look up "argument from incredulity" on google, and hope that she has enough of a conscience to think her decision through.