Ok I really do not know since I have always have a PIN, and hitting install/agreeing to terms. I could see given she “closes” those notifications of install (install, later, more info) sometimes “ignoring“ she thinks she is doing she must be hitting later thinking it is same as no. But never had a 11.x or 12.x install happen with out the manual intervention to complete. Usually it probably downloaded and could not install (error messages to same happen) and then I would decide if she needed it ;-). But the install (download already there or done during night ) happened during middle of night with out anyone there. She picked her phone up and it was at the welcome screen requiring the new setup things from language to security and such which I usually handle so the update had occurred.
She probably accidentally clicked to install the update later that night. As others have said, some user input is required before an update occurs. Likely she meant to click ignore or just didn't fully read and selected to install the update later.
Once that's done, if it's plugged in to charge overnight and connected to wifi the update can occur without any further input.