Most weight that is lost early on is water weight.
You can lose a lot of weight and gain muscle in a month. But it takes an incredibly intense workout regiment and diet to do so.
Mark Twight, who trained Henry Cavill and got him in shape for the Man of Steel movie, had a client/friend who dropped 30 pounds in a month...but that guy was on a crazy workout and ate something like a strict 1,200 calories a day