How do I identify the device where the photos came from?
You can group the photos by the camera that took them, using Smart Albums.

First click on one of your girlfriend's photos and Show Info under the Window menu. You will see something like this. It's the camera model that is important (I have boxed it in red).



My hope is that your iPhone is not the same model as your girlfriend's. If it isn't we are in business. Assuming that they are different here is what you need to do.

Go to File/New Smart Album...

Call it "My Girlfriend's Photos" or whatever (I called mine "iPhone 6s pictures"). Set the Camera Model to include what you saw in the Info pane. In my case, it's iPhone 6s. See below.



If you show the Sidebar (View menu) you will then see your Smart Album somewhere in it. Here's how it looked here.



If you are showing the Sidebar, and you click on "All Photos" under the heading "Albums" (not "Photos" at the very top), you will notice that View/Sort will say something like "Keep sorted by oldest date added." This may give you a second chance at figuring out which photos are your girlfriend's, because the sorting is by date ADDED and not date "taken." So the pictures that were newly added would be at the bottom, and those are probably your girlfriend's. You could select them and put them into a regular album and you'd be all set.

Personally I am hoping that you two have different iPhone models because the Smart Folder method would be very cool.