Personally, I've always been baffled by the use of the term girlfriend when refering to somebody who was actually just a friend of somebody female, who also happened to be female. The logic thereof escapes me.
Obviously, if somebody is your girlfriend, then there should be something more there than merely "friendship".
Which, in it's own way, likely leads us (as long as we're worrying about semantics) to wonder about the true nature of what differs, truly, between being a friend, and being (to be less technically obfuscating) a significant other.
I think that the usage of the term "girlfriend" to refer to the friend of a girl who is also a girl may be cultural, in the same way that hugging a newly met stranger on one coast is acceptable, whereas on the other it's a good way to get curb hopped.