You cannot prevent the Photos.app on the iPhone or the Mac from scanning the photos for faces.
And from trying to recognise the same face in other photos. But the Photos.app does not name the recognised faces. Someone has to assign a name to the first detected faces to make it possible for photos to find more photos and to add a name n all photos.
To remove your name from the photos of you, ask your girlfriend to erase the name from the People album, so the face will revert to "Unnamed". Additionally, she can remove the album from the People album.
Apple says "privacy build in" and so on, but is this truly private? And how do I revoke my face being recognised, so it doesn't happen again and delete my face from Apple servers?
Your face in the People album on her iPhone can only be accessed by the owner of the AppleID.
Previously the Photos.app did not sync the named faces with iCloud at all. because of privacy reasons the face detection has been done on all devices separately. And the named faces have not been stored in iCloud. It started only with iOS 11 and macOS 10.13, that we could sync the named faces between all our devices. Many users have requested this feature, because they found it annoying and wasteful to have to name the faces on each device separately.
