Used when referring to a third party with an as of yet unspecified or ambiguous gender when such a distinction is critical to the context of the conversation.
Guy A: So my friend Alex wanted me to come in the store to buy tampons but I was like no way!
Guy B: Hold on are we talking guy Alex or girl Alex?
Guy A: Sorry, my girl friend Alex wanted me to ...
Friend Who's A Girl - used in texting and blogs when speaking of a member of the opposite sex (opposite of male in this case) who is a friend but not a girlfriend (GF). ("FWAB" is not used by females; "guy friend" is used instead.)