Hi Elisabeth, my name is Neil and I'm a Windows 11 user like you.
In my Windows Photo's App (up to date), I cannot right click and rotate my photos either. The option does not exist. The only similar option is Resize.
In fact I have never been aware that you can do this in the Windows 11 Photo App.
I do know that in the previous Windows Photo Viewer (different App in Windows 10) that this was an option, but never in the Windows 11 Photo's App?
I would be interested to know if your girlfriend is actually using the same Photo App, and if so, what version is she on? You can find this out by clicking the 3 dots (...) on the Photo App toolbar and then clicking on 'Settings'. Scroll down to the bottom and you will see the Photo App build number. Mine is: Microsoft Photos 2021.21120.8011.0
There are other ways to rotate photos though in Windows 11. In the Photo App you can press Ctrl + R to Rotate any image. I think this is the same as using the Photo's App rotate button though and may change your metadata, although I'm not 100% sure about this. You'd have to check.
Another way is to open File Explorer and navigate to any folder with photos. Highlight a thumbnail of a photo and you will see options appear to 'Rotate Left' and 'Rotate Right' along the toolbar.
Clicking one of these options and you will see the thumbnail rotate in the direction you selected, and this also rotates the actual photo too. This may not affect your metadata.
I hope this gives you options and are able to rotate your photos as you want.
Let me know how you get on.
Kindest regards,
Neil