Fun idea. That could be called hyperlapse.
If you use a specific memory card for that project, you can always switch from viewing the first photo of the project to live view.
Another option is that you built a box with a grid or something and put it in front of your camera, like the old matte paint technique used in cinema. Instead of having a paint of a scene, draw a stick figure as a reference, remove it and take the shoot.
After this you can use an image stabilization software. You can use Blender which is free. https://www.google.com/search?q=blender+image+stabilization
Level your camera with a feature of your girlfriend, like the tip of the chin and shoot at a wider angle that you think looks nice, so you have something extra space to crop and adjust.
You could do a very complex stop motion animation if you prepare each pose in advance, and carry it as a reference.
Study hyperlapse techniques https://www.google.com/search?q=hyperlapse here is a very good tutorial: https://www.youtube.com/watch?v=DcylVx2ex78 and they are using a monopod.
I feel there is a chance the effect won't be noticeable because the lighting and obviously the background will be very different, so focus the framing on her. I do not mean center her, but compose based on her.
An additional thing that could help "balance" the images is the use of an external flash, so you can put the flash in the same relative position and this would give you an additional stable feature - the source of light.
To smooth things even further do not take just one shot. Take several on the same location with some differences, probably a "circular dolly", where she rotates a bit facing you.
Now you have an additional element to smooth the result. A camera movement.
And an additional thing. Let her use some clothing that can be reused again an again. A t-shirt (or several), a quid, a cap so she is dressed similar to enhance the effect.
Share a link after you finish your project!