They will work, but he will have a lot of lead to deal with in the rifling. Unless he is going to hunt with them, I wouldn't recommend using them. He would have to have a good lead removing solvent, brushes, patches, and a ton of elbow grease. If the barrel has a cantilever scope mount, you can bore sight a scope by looking through the barrel and adjusting the cross hairs accordingly. That will save him the aggravation of a ton of cleaning at the range. The sabots need a clean rifled bore, not one full of lead.
With that said, if all his shots are under 75 yards, why bother with the sabots at all? I have killed several deer with foster slugs through a rifled barreled 12ga out to 100 yards, but you will have leading to deal with. I use a 20ga Ultra Slug hunter with the high performance saboted slugs now, but if you can only shoot 50-75 yards, the foster slugs will do fine at those ranges through a rifled bore.