whacker,
First, what type of shotgun does she have, rifled or smoothbore? does she have a scope mounted, open sights, how far is she comfortable shooting and what type of game? Deer, Elk, Moose, Pronghorn or Bear? That will be the biggest determining factor in slug selection.
The saboted slugs are very expensive but worth every penny in accuracy if she has a rifled barrel.
My 2006 Colorado Big Game Brochure (Guide) says:
5. SHOTGUNS
a. Must be min. 20 gauge, and fire a single slug.
b. Barrel length must be min. 18 inches. Min. overall length, 26 inches.Remember this is only a guide, you can find copies of wildlife statutes and regulations on their website,
www.wildlife.state.co.us or at any DOW office. You may want to look into the use of saboted slugs, as the 20ga uses a .45 caliber slug in the 260 grain arena and the 12ga uses around a .50 caliber slug or bigger in the 385-400 grain arena, depending on brand. The full bore Foster slugs go from 5/8 oz to 1 oz in the 20ga and 1 oz to 1-1/4 oz in most commercial offerings for the 12ga. Don't overlook the Rottweil Brenneke slugs, they are very accurate in my sons 870 and are devastating on whitetail deer and IMHO are a far better choice than the Rem, Fed & Win foster slugs. There are some custom slug manufacturers such as Dixie that offer hardened heavy slugs that may be better suited to your BIG GAME, but these are not for the recoil shy.