Have her try some Montrail Sandia Peak boots ($120), a boot designed to fit a woman's foot. It's not a real rugged boot, but for moderate backpacking and day hiking it's fine. Take a look as well as La Sportiva's Women's Storm GTX ($190), which adds a waterproof liner to a good all-around hiker. And L.L. Bean's Leather Cresta Hiker ($165) is a book that seems exceedingly accommodating fit-wise. It comes in women's sizes, and in narrow widths.
Of course, it could be your girlfriend simply falls at the far-left end of the bell curve, with a heel that's narrow even for a woman. In that case, you may have to resort to several tricks to get the boot to fit. After-market insoles are one option; Superfeet insoles ($30) are available in high-volume models that soak up some extra space in the boot. You also can buy inserts that wrap around the heelcheck with a footwear store for such a thing. But my guess is that the right boot will go a long way toward solving the problem.