RE: Old Lyman
I've got a Lyman Great Plains Rifle. It's one of the newer ones with 1:60" twist, and it's definitely a roundball only shooter. Not to say that's bad...I took my first blackpowder deer a couple years ago with it, and it did the job quite well. My load is 67gr of FFF Goex with a .490 roundball (pillowticking patch). Not sure if that will match up well to a shorter barrel with a faster twist, but it might be something to try.
Regarding the brass buttplate...I was taught that with that style gun, it was designed to not go directly on the shoulder. Rather, when you bring it up, it would go in the crook between your shoulder and your upper arm. The reasoning behind that goes back to the original design, which the GPR is a pretty good replica. It was designed for shooting while wearing heavier clothes, and allows for a more comfortable, faster, and more repeatable firing position. Make sure that when she starts out with it that she starts with some lighter loads to get used to the gun before building up the powder. Also, she'll take much less beating if she's shooting it from any position other than at a sighting bench. At a sighting bench, the tendency is to not pull it as tight into the shoulder, and the body tends to not move with the shot to soften the recoil some. I remember the first time I shot mine off a bench I had a black and blue for a few days from only shooting 15-20 shots. Since then I figured it out and it's been much less painful.