Well this probably would work, if the reservation is in your name. Your girlfriend however, would only get a bill in your name, so I don't know how that would affect he getting reimbursed.
It all depends how check-in goes. She could say that she was your wife and you were coming in on a flight later in the day. When you make the reservation online, you could add her name to the reservation and this should take care of check-in problems.