By default, you should each have your own Opera and your own Opera settings.

You:

Install folder: "C:\Users\yourusername\AppData\Local\Programs\Opera"

User data folder: "C:\Users\yourusername\AppData\Roaming\Opera Software\Opera Stable"

Cache folder: "C:\Users\yourusername\AppData\Local\Opera Software\Opera Stable"

Her:

Install folder: "C:\Users\herusername\AppData\Local\Programs\Opera"

User data folder: "C:\Users\herusername\AppData\Roaming\Opera Software\Opera Stable"

Cache folder: "C:\Users\herusername\AppData\Local\Opera Software\Opera Stable"

Or, if you decided to install Opera for the non-default option of All Users, it'd be like this:

You:

Install folder: "C:\Progam Files\Opera"

User data folder: "C:\Users\yourusername\AppData\Roaming\Opera Software\Opera Stable"

Cache folder: "C:\Users\yourusername\AppData\Local\Opera Software\Opera Stable"

Her:

Install folder: "C:\Program Files\Opera"

User data folder: "C:\Users\herusername\AppData\Roaming\Opera Software\Opera Stable"

Cache folder: "C:\Users\herusername\AppData\Local\Opera Software\Opera Stable"

(Replace "Opera Stable" with "Opera GX Stable" everywhere and the "Opera" program files folder with "Opera GX" everywhere above if you're talking about Opera GX instead of Opera One.)

Either way though, you each would have your own user data and cache folders that would not interfere with each other at all.

So, as Leo said, check the paths in both your Opera and her Opera at the URL opera://about and compare.

So, either something's wrong with your setup or on both your Opera and her Opera, you're logged into the same Opera account for Opera Sync and you're syncing settings.