I'm sure you could pay a webcam chick enough do to just about anything (including holding up a sign with various stuff written on it). So I'm not sure there is a way to "prove" this is his girlfriend or not.
At this point of the thread, I'm getting bored with the same old poses in different underwear. I'm with the others that have asked for full nudity, fingering, toys or anything besides the thumb pulling at the waistline of her panties.