Back on topic. I just started loading cast for my 45-70. I chose the Lee 340gr and am very happy with it. The load is not mild but still kicks less than a 300gr HP at 2100 fps. (Marlin 1895 Guide Gun)
I'm sorry but I simply don't see a smooth transition for your wife here. She's going to have to withstand a lot more recoil and deal with a lot more trajectory.
I say buy her a box of bullets and let her continue on.
Motor