It is definitely a fired civil war type of bullet. That type is called a Minie ball ...named for the inventor of that type, Captain Minie of the French army. The specific variety of Minie ball your friend's sons found is a yankee-made 3-groove minie, manufactured for use in the US muzzle-loading .58-caliber Springfield Rifle. (Sidenote: Although that bullet actually has three grooves encircling its body, not raised rings, most civil war relic diggers call it a "3-ringer.")
I should mention, other varieties of Minie balls have two body-grooves, or one, or none. And others do have raised rings. The presence of a large cavity in the base of a muzzleloading bullet is what makes it a Minie ball, not the shape of the rest of its body.
I'm certain the boys' bullet is a fired one because its lead body shows "rifling-marks" (axial ridges), which are created by the gun's rifling-grooves during firing.
Before firing, that bullet typically measured about .57-inch in diameter, in order to be loaded into the muzzle of the rifle's .58-inch diameter bore. Firing expanded the bullet to .58-inch, and 100+ years of lead oxide patina buildup adds another .005-to-.01-inch to its diameter.
The bumps and gouges on it were caused by high-speed impact with the ground after firing ...not by the "eager boys" digging. If the boys had hit it with the shovel, you'd see gray-lead areas on it where the shovel knocked off the white-ish lead oxide patina.
There are two possibilities for how that yankee Minie ball came to be fired in Illinois.
1- Newly recruited civil war troops were given training & target-practice in their home state, before being shipped off to the war.
2- After the war ended, multi-thousands of civil war rifles, ammo, and bullet-casting molds were sold to civilian buyers as "war surplus," for use in hunting. A legendary example of that is World War One hero Sgt. Alvin York, who reportedly used a civil war era muzzleloading rifle for deerhunting as a young man in the mountains of Tennessee during the very-early 1900s.
Update: After I clicked "post," I see that Kieth-TX and Tennessee Digger gave you short (but correct) replies while I was typing my very lengthy answer. Since you didn't know what the bullet is, I figured you might like some educational info about its type and its history.
