Remove the devices from FindMy.
But that's not your real problem. You bought a Mac improperly setup for resale and the previous owner is being uncooperative. Actually I don't blame her for not giving out her ID and PW.
That notwithstanding, both of you have contributed to the problem.
Start by attempting to sign her out of her Apple ID. Generally it takes no password to do that. If successful, signin with your Apple ID and password.
Once signed in with your Apple credentials, attempt to do what you haven't been able to do before. But if passwords are embedded in the app or process, you're no better off there and need her input.
You can try and erase the drive, but I've never tried that on a machine I didn't have full ownership of. Or at least have the owner available to enter PWs.
Not properly setting up a Mac for resale before ditching it is never a good idea, as is buying one not properly prepared..