How to unlink previous device owner's keychain data from my Apple ID

My business partner recently gifted me an older MacBook when she upgraded hers. She'd removed all her files but we didn't factory reset the machine right away. I logged in with my Apple ID and used it for a while as a backup device (I have another primary MacBook) before finally doing a factory reset. Since logging into my Apple ID on her machine, I am seeing all of her iCloud keychain data on any device where I'm logged in, and can't figure out how to remedy this.

Is there a way to remove her keychain data from my account without manually deleting every password that appears to be hers (there are hundreds, and some are not easily identifiable as hers vs. mine, e.g. if the username is an account number and the password was auto-generated). If I do manually delete passwords, will I need to do this on every device or will updates sync across all devices?

Any help at all is appreciated! Thank you!