Have you looked at Terrys? They make a 16" frame size that
might fit your girlfriend perfectly. If she's 5'0" with short legs,
I suspect her inseam is less than 28", since I am 5'2" with average
legs for my height, and my inseam is 28"! I ride a 17.5" Terry
frame and a 46 cm frame on my new Aegis Swift. The 17.5" Terry is
the next size up from the 16" frame. The 16" frame is very short.
A woman in my bike club who is about 4'10" or 4'11" rides it
and loves hers! www.terrybicycles.com.

Good luck and let us know what you decide!