looks like that "simple" shifter is simply WAY overshifting on the 3-2-1-2 move.. maybe it's time to remove the "simple" and replace them with trigger shifters from Shimano.... or hope to get a microshift thumbie that works better than the defective one on your wife's bike. ;-)

i'd go with the Shimano triggers and not worry about them for a decade of more....many pairs available for under $40, shipped... installation not included.

also... do a careful inspection of her Cassette to see if there is any deviation from flat/true.... even a single bent tooth can cause havoc.