"Cambria" is not originally a personal name, but is a Middle Ages Latin version of "Cymru", the Welsh word for the country of Wales. In its root form the word is "Cymry", meaning "the people". You'll have to trust me that the difference in pronunciation between Cambria and Cymru isn't a major stretch.
Either your friend is of Welsh descent or her parents just liked the sound of the word. In either case, it's not common as a first name even in Wales.