I prefer not to hide my iphone 4's sleek looks.
I use a devilphone back skin and a rocketfish frame bumper.
At the moment I dont use a front shield as I'm not keen on the difference in feel on the screen.
Stay clear of Zagg. They are notoriously hard to fit and give an orange peel look. Thats my own experience. On balance some users have reported no issues in fitting and like the orange peel look.
Hope your girlfriend enjoys this awesome phone.