She tends to stop pedaling, and crank it from limit to limit, which may have contributed to the recent cable failure.
This reads like you should fit a internal gear hub on her bike rather than a derailleur, they work whether pedaling or not, even at full stop.
derailleurs only work with moving chainrings, so rather than force your marriage
into distress teaching her what must be done to shift a derailleur bike,
get it rebuilt around an internal gear hub.