Hmmm, 10 years old is old in computer terms. You didn't mention what OS is she on currently and if she has Malwarebytes installed and what browser she is using now? You can stretch a decade old machine a bit more by installing Malwarebytes and paying for subscription and that will help prevent older Macs from being attacked by malware and ransomware, which are more common today than a virus infection. She can also install Brave or Microsoft Edge as both browsers still support older machines and OS. Having the latest browser will allow you to bank safely since the bank website will test the version of her browser for safety before it will allow her to bank through https. And tell her not to download anything sketchy that's too good to be true and she should be fine.
My dad is running my 2009 17" Macbook Pro with Linux Mint 20 (not a Mac OS) with Brave and does online banking and email perfectly fine and safe since Mint 20 is a modern OS and supports the latest Brave browser.
If she can afford a new computer, then it's probably best to get one of the new iMacs. But if you're on a shoe string budget like my dad is, then a little bit of software upgrading can renew an old computer and use it for today's online banking.
Hope this helps.