12 Gauge standard 2 and three quarter inch brenneke slugs. Buckshot is not reliable and does not penetrate well on big grizzlies. The gun is to handle the
definition of extreme shtf.
Synthetic standard shoulder stock, to fit her. Most women need a shorter length and different pitch to fit their shoulder shape and arm length. It must point where you look, especially if you are not too experienced.Bead sights are fine, with practice. Standard forend. Maybe an extended tube, but watch the balance-don't want it too heavy. You want minimum five ####s( oops I meant shots, but they often happen simultaneously in a grizzly encounter, not that you notice til later

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Practice and familiarity with the gun is crucial. Start with a 22 til she can hit, then several hundred birdshot through the shotgun until she has fun and knows the gun.Totally irresponsible, and inneffective otherwise. If she thinks she won't need a defensive system, and isn't prepared she should not be guiding, armed or not.Negligence is easy to show after something has gone sideways. How many of you would be comfortable OR FEEL SAFE around her while she is carrying her shotgun? Short ones are easy to point inadvertently, even if you are experienced.
You must know bears and bear behaviour, and unless you understand that thoroughly and can judge when shooting is an appropriate option, forget it.Too many dead bears, not enough safe people, too much ignorant attitude. A gun is the last thing, although an important back up to brains and smart behaviour.
If she is mostly on horseback that reduces risk.She has no advantage at all with a pistol grip, and does not need the reduced length and weight on a horse. Read Herrero and Gary Shelton.For someone like this, pepper spray will be more appropriate for safety and effectiveness. I hope her horse is smarter than she is. Most women with training are better at the right attitude than men, who tend to get too macho, as if it were a substitute for astute behaviour, and train faster because they don't have to lose all that attitude first. There might be hope.
I know this stuff because I teach it. Different perspective.