Well, there was the fat and saucy, round heeled tribe of semi-hideous looking hookers from Brooklyn that I used to routinely converse with at dawn in Times Square circa 1974. My friend and I always told them that we were too expensive for them and that they couldn't afford us. Remember one of them saying to me one hot August morning that she'd provide me with a real girlfriend experience if I'd only let her and pay the requisite fee.
But, somehow I doubt that this hooker was the origin of the term. Probably have to go back to the 1920s or 1930s when men first started having "girlfriends" and women started becoming "liberated."