Since he' s going to be a guide, I' d say go with a .416 rigby, .45-70, or a .458Lott/Win. I' d suggest some other rounds if it were to just be a hunting rifle, but since it' ll be in use in defense of life more often than in the hunt, I' d be more comfortable with the larger bullets and increased power of these rounds (.45-70 excluded). I include the .45-70 because, although it may not have a ton of velocity borne energy, it still has a massive bullet face, and lever action rifles handle very quickly, they get on target faster due to shorter length and will lever out rounds more quickly. THe .458Lott/win combo is a great choice because it gives you an option of two ammo types, both capable of doing the deed at any range.