I suggest you click on Page 2 of this specific forum-area of Findmall, and scroll down until you get to the thread titled "Here's the cannon ball, Dave" (started by "don in ma"). He recently dug a 15-inch solid-shot. In that thread, he got some good advice for what to do (and not do) about getting it out of the ground. : )
I know about Vicksburg artillery. The biggest cannonball used there was a 13-inch Mortar shell (and from your description, that's what your friend has found). It is QUITE valuable ...so yes, it's worth digging.
AND, please advise him to NOT "publicize" his find. People used to say "Loose lips sink ships" for a reason! : ) The less folks around Vicksburg that hear about your friend's shell, the better off he'll be.
The standard weight for a civil-war 13-inch Mortar shell is 210 pounds.
Your friend's shell was fired from a yankee "Mortar Raft" (quite large) anchored in the nearby Mississippi River. (The Confederates did not have any 13-inch mortars, so that shell HAS to be yankee-fired.)
Lastly... though it is indeed an explosive shell, your friend need not be concerned about it exploding while he's digging it - or moving it home. The 13-inch Mortar shell was made to use a large simple wooden fuze-plug, which has almost certainly rotted and leaked groundwater into the shell's powder-cavity. So for the moment, moving the ball is fine. In all the history of metal-detecting relics, not even once has a CIVIL-WAR shell blown up while being dug ...or even while being transported. I repeat: Not a single one, in tens-of-thousands of diggings and transportations of CIVIL-WAR (and earlier-era) shells.
Your friend is far-far-far more in danger of getting a serious hernia from his discovery than a boom.
The "Bang" problem comes later on. More than one or two people have come gotten a civil-war shell to explode by:
(1) putting it in a hot oven or a fire, or
(2) drilling into it to try to deactivate it.
Do NOT inform the police or other authorities. They'll confiscate the shell. For deactivation, contact a relic-digging professional deactivator. There are some of those available. I'd do the deactivation for you, but I'm a loooong way from Vicksburg. ;-)
Regards,
TheCannonballGuy (Pete George)