This doesn't sound good. Regarding my friend's gun, although the fit problem as described to me sounds like its an asthetic one, I'm wondering if this mis-matched slug gun is even safe to shoot. He said there is a slight gap between the barrel and the receiver, and the barrel is also shorter.
If their is a gap between the reciever and the barrel, he should definitely NOT shoot it. Have him take it to a smith to see what is going on.