Using the best slug in each, the difference between my Deer 870 with its rifled choke tube and my HD 870 with its chokeless bbl is less than an inch at 100 yards. YMMV of course.
The Rottweil Brenneke 20 gauge slug is a great deer round. Others are OK. Find a slug that puts them into a deer shoulder at 50 yards and you're set.
If your friend is looking for a rifled tube because the piece isn't shooting well, there's old threads on setting up pump guns for Slugs that may be invaluable.
HTH....