Do you trust you gf though? She wasn't too drunk to get out of your bed but too drunk to get out of the other? In her defence though, she was there first, so your friend should have found somewhere else to sleep. Did he know there was another room with an empty bed?
Your friend has to get most of the blame there. I'm still undecided about the gf.
All this could have been avoided if you guys didn't drink that much.