I wouldn't say at 14 weeks he was being "vicious' about it..As you said, he was getting amped up, and probably didn't like ending the "game" , so he latched on..
Teething can also 'hurt'.
I probably would have scruffed him myself, I would also start teaching a good 'leave it', he puts his mouth on a human, work on him 'leaving it'..redirect.
I think you mentioned he's doing really well otherwise, so I wouldn't worry about this incident to much (even tho it sounds like your GF got nailed good, puppy teeth are brutal)
He's a cutie by the way