Is your friend going to be pushing deer? If not, there's not much reason to bring an autoloader. An autoloader is just going to jack up the price.
A good shot placement with today's modern sabot slugs will generally not require any follow up shot. On that note, one of the best bets for both accuracy and price is an H&R Ultra Slug Hunter (a break action single shot). Another good slug gun is the Savage 210, or the brand new 220. This shotgun costs much more than the H&R, but is a bolt action repeater. Remington also has a special version of the 870, designated the Super Slug, which has a pinned barrel and an optics rail on the receiver. This design is apparently superior to cantilever barrels, which have degraded accuracy past 100 yards due to harmonics. That being said, cantilever barrels are a good option for shotguns with exchangable barrels. The Remington 870, 11-87, Mossberg 500, 535, 835, and 935 are affordable shotguns that have cantilever barrels available.