A 500 doesn't need a slug barrel to fire foster type slugs any more than any other shotgun out there. You can fire a slug out of any modern smooth bore barrel and not do any harm what so ever. You can fire sabot slugs as well but you will not get any kind of accuracy.

The 835's must have a dedicated slug barrel installed because the bore of the 835 smooth barel is .775" inside diameter vs. a standard 12 ga barrel being .729" inside diameter which is also the diameter of the 12 ga slug. If you fired a slug in the .775" barrel it would rattle down the barrel and probably damage the barrel, gun, or you.

Now on to the original post. For a shotgun barrel to split down the side like that it......... 1) was previously damaged in the area of the split. 2) had an obstruction in the barrel such as the previous slug not exiting due to an under charged load. 3) your friend fired a double charged load. 4) The shell was improperly assembled when loaded.

Any of the last three can send the pressures inside the barrel high enough to split the barrel. A shotgun barrel is only designed to hold in about 10,000 - 15,000 psi.

It is an incredibly remote chance that the barrel had a manufacture defect but that possibility is extremely remote.