Wow. The shit I read on a daily basis astounds me. First off, NEVER get a chick a revolver. EVER. If she's inexperienced, I ASSURE you that with option A. (double action shooting) she's going to miss the target with every round in the cylinder, and with option B. (single action shooting) she's going to drop the hammer before the gun's on target. Typical revolver sights suck as well.
And a SHOTGUN is the most effective weapon you could use without training? You've got to be kidding me! The poster said GIRLFRIEND, not infantryman lol. A woman shouldn't even walk past a shotgun, without weeks of dedicated training. No way in hell. And a shotgun is NOT a point and shoot weapon. It has to be aimed just like everything else. Ever watch an average, untrained woman try to shoot a 12 gauge? It's like a 15 year old trying to fly a helicopter. Total disaster. One shot from a shotgun indoors is going to destroy a chicks hearing soo bad she's going to be all but disoriented after the first shot. And what if you miss. Do you realize where that slug or buckshot's going to end up? Out of your house and into the neighbor's. Don't do it. Shotguns and revolvers are not for the inexperienced.
If you want a gun that will work every time, and you don't have to fumble with safeties at 03 in the morning when you're tired, caught off guard and scared shitless, (especially on a really nervous woman) get a glock, or a glock type of weapon (m&p, xd etc.) and put a light/laser on the rail. Show her how to pick it up, point it out in front of her, switch the light/laser on, ID friend or foe, and pull the trigger. Zero the laser to the typical distance she would have to engage someone at her house at. Then take her to the range and practice. Shoot an entire box of your favorite defense hollow point to check for function, feeding, recoil, and point of aim, and then shoot ball the rest of the time.
Stick with 9mm; it's cheap to buy by the case, doesn't kick that much, is sufficient to put down a potential attacker, and everyone carries / uses it.
Trust me, stay away from revolvers and shotguns. Everyone perpetuates these myths because they've heard someone they trust or look up to say them. They take more training than you think. Just because you can pick up any revolver and shoot it ok, doesn't mean your woman can. I've seen, and trained MANY women on a revolver, and NOT A ONE of them hit ANYTHING with them. Women just aren't built with the same set of traits men are. They're weaker and slower, and a revolver or a huge shotgun is the last weapon they should be utilizing.