Does she habitually turn the ringer volume all the way down before going to sleep? On most Android phones, if you lower the ringer volume one step below mute, it will activate vibrate mode.
I am having the same issue -- I put my phone on silent every night, and it is set to a low ringer volume when I wake up. Started after installing the 26A update last week.
I had the same issue with vibrate switching on at night. Check scheduled settings under quiet mode menu. Mine showed as being switched off but when clicking into that setting it showed as being on for M-F. I unchecked each day and switched the setting to off which fixed my problem.