The way I understand Subaru's air bag deployment, there are 3 sensors. An acceleromter to detect sudden stops (crashes) to deploy the bags, a position sensor in the drivers seat to adjust the speed of deployment (the closer you are, the faster it deploys) and a weight sensor in the passenger seat that allows for distinguishing between a heavy (>100 lbs) passenger or a lighter (<100 lbs) passenger. The bag still deploys regardless (I thought), but the sensor adjust the deployment accordingly. I might be wrong though. To the best of my knowledge, these can't be adjusted.
IMHO...these are designed this way on purpose. As long as your girlfriend is wearing her seat belt, she will be fine. For someon short and lightweight, an air bag deploying is not always the best thing due to the angle of deployment and the rate. These things can easily snap a neck of a lighter framed person.