Sounds like a porch light, and peep hole would have been a great asset to have in the situation the OP described. I know that in my younger days I lost my keys when I went for a hot night in the old town so say. Though I lived alone, and kept a well hidden key to the front door. I did not have my car keys with me due to the fact that I took a cab becasue I knew I would be drinking. I can see a similar thing when one has a roomie.
In short I wold suggest finding a way to identify who is at the door, and teaching the girl friend how to handle the weapons in the house as well.
No matter how many times you do it and nothing happens it only takes something going wrong one time to kill you.