The way I would describe this would be to make another comparison to thinking in general. Do you have an internal narrator, or have songs get stuck in your head? If you do, you are thinking with your "mind's ear," so to speak. If you are at all familiar with this concept, even if you can't imagine absolutely anything you want to hear, it's a great analog for what it's like to use your "mind's eye." In the same way you don't literally hear what you think, you don't actually see what you think either. You just use those parts of the brain to create the sensation and experience it in some way. It doesn't overtake your primary vision and literally activate photon receptors in your eyes, but it can distract you from that sensory information since you're using that area of the brain.

Really I am interested in how literal your girlfriend is there, because if that's not a miscommunication, that just sounds like on-demand hallucination. I could clearly imagine something in front of me. I could manipulate it, I could imagine any of my senses to interact with the object, but at no point does it appear to literally exist in the world as if it's a hallucination. That would be an insane ability to have, and I don't think that's what people generally mean when they use their "mind's eye."