Ok, so I totally misread the question.

Bulb mode, get your exposure right for the stars behind. Once you have this, set the shutter open for the required time, put the lens cap on / something over the lens (you'll have to count the time, the shutter needs to stay open).

Have your girlfriend stand where you want here, charge the flash and set it for the power you want. Take the cap off and fire the flash.

If you really want to have some fun, you could 'paint with light' on her using a torch to illuminate what you want. It takes some trial and error but you can get some fun effects.

==================== old answer One way of doing this, depending on the scene, is:

• Use a tripod
• Set your flash to 2nd curtain mode (so it fires just before the shutter closes rather than just as it opens)
• Set your exposure settings for the background so you get the detail
• dial down the flash power to an appropriate setting (this might need some trial and error to get right)
• Tell the model to keep still
• Take the shot!

The shutter will stay open long enough to get in some light from the background and the flash firing at the end of the frame will illuminate your girlfriend, because the flash is a short burst of light it will freeze any movement in her so you don't have to worry about losing sharpness in her features.