Like the woman who called in, I had a new Opel. It was a Kadett. Once after leaving the dealer for a tune-up, the stick shift came out of the floor. I quickly reinserted it and drove back to the dealer.
Long ago, I had an '81 Chevette which would pop out of reverse (a combination of soft OEM motor mounts and my aggressive driving). To compensate for this, I began to hold it in the “slot”. Eventually, the shifter popped out of the linkage. The pin or linkage prematurely wore out. I repaired it myself several times. Eventually I wised-up. I learned to back up more cautiously and stopped holding the shifter in place.
It is possible that something similar could be the cause of the woman’s history of shifters coming loose in her hand.
Your suggestion that she only shift with three fingers is a great idea. Also, not to rest her hand on the shifter and not to hold the stick in place. If one operates a manual transmission, they should learn to gently shift, primarily with the wrist. They should practice this until they can shift smooth as butter, not “hitting the sides of the slot”. They should continue to practice this until they area able to do so.