I have taught women smaller than her to shoot an 870 well with full...not reduced...#00, but they were required to do it in order to graduate and most were pretty athletic. One big issue is stock fit and how they hold the gun. They MUST hold it TIGHT to the shoulder and bring the gun UP to the head / cheek, not vice-versa. Again, this requires a shorter stock for most. Many also believe that the full butt of the gun must contact the shoulder...not so...you can do very well, indeed, with half, or less, of the butt above the shoulder "pocket" so long as you hold that sucker tight.

Take a 2x4 and round off the edges, then throw on a recoil pad...that is essentially what a shotgun stock is. Take that 2x4 and allow it a "running start"...i.e. not holding it tightly to the shoulder... and it will smack the shoulder pretty darned hard. Hold it tight and it will give you a heavy "push" instead. With a too long stock, every time the action is worked, the shooter will tend to push the stock away from the shoulder a bit because they are trying to sight down the barrel...guess what? Recoil.

Still, there is no way that you can change physics. A light, short barreled shotgun with some full Buckshot or slug loads can generate up to 50 or more ft. lb. of free recoil...if that is a lie, I have heard it several times in Instructor schools and from Tommy Campbell at S&W, who says they measured it there in the 1980's. Perswonally, I believe it. If you weigh 200lb. and shoot a big bore rifle generating close to half your body weight in recoil, it is not gonna be a real fun ride, is it?

Get her full body weight behind the gun as well. A "Boxer" stance with the front (weak) knee bent and the back (strong) leg straight, with around 65% to 70% of her weight on the front leg is very helpful...she needs to "get into" the gun, not try and "avoid" it...which is very common with new shooters. Definitely get the R3 pad...they are good. As far as some of the other recommendations, I have no direct experience with them.

Good Luck