I have a friend who's son (a small young man) hunts with a Knight Wolverine .50 caliber. He shoots 80 grains of Pyrodex RS and a 240 grain XTP and kills his deer every year. If that kid could handle that rifle, any woman could.
Actually I think almost any of the modern rifles could be loaded down enough for your lady to shoot one. Muzzleloaders don't seem to kick all that bad until you get them over 100 grains.