You should always expect the least probable thing. My father managed to trigger entry to diagnostics mode (Kindle has Coward's rescue pack installed) in first 5 days of using it. While I in 6 years of owning Kindles have never had Kindle discharge completely, so I didn't even consider warning him about that, to not try to power it on while it charging.
Fortunately he didn't try pressing any of the menu options in diagnostics
After that I've made sure to show him how KUAL looks and how to exit from it and warned him not to press any off the KUAL buttons. I've also changed the Home view filter from "All items" to "Books"
Also had to remove all saved WiFi SSIDs to avoid having the firmware updated because somehow my father keeps turning off Airplane mode.